diff --git a/src/core/server/config/deprecation/core_deprecations.ts b/src/core/server/config/deprecation/core_deprecations.ts
index 6a401ec6625a2..36fe95e05cb53 100644
--- a/src/core/server/config/deprecation/core_deprecations.ts
+++ b/src/core/server/config/deprecation/core_deprecations.ts
@@ -97,7 +97,6 @@ export const coreDeprecationProvider: ConfigDeprecationProvider = ({
 }) => [
   unusedFromRoot('savedObjects.indexCheckTimeout'),
   unusedFromRoot('server.xsrf.token'),
-  unusedFromRoot('uiSettings.enabled'),
   renameFromRoot('optimize.lazy', 'optimize.watch'),
   renameFromRoot('optimize.lazyPort', 'optimize.watchPort'),
   renameFromRoot('optimize.lazyHost', 'optimize.watchHost'),
diff --git a/src/core/server/http/http_config.ts b/src/core/server/http/http_config.ts
index 7d6a6ddb857b1..73f44f3c5ab5c 100644
--- a/src/core/server/http/http_config.ts
+++ b/src/core/server/http/http_config.ts
@@ -42,15 +42,6 @@ export const config = {
           validate: match(validBasePathRegex, "must start with a slash, don't end with one"),
         })
       ),
-      defaultRoute: schema.maybe(
-        schema.string({
-          validate(value) {
-            if (!value.startsWith('/')) {
-              return 'must start with a slash';
-            }
-          },
-        })
-      ),
       cors: schema.conditional(
         schema.contextRef('dev'),
         true,
@@ -149,7 +140,6 @@ export class HttpConfig {
   public maxPayload: ByteSizeValue;
   public basePath?: string;
   public rewriteBasePath: boolean;
-  public defaultRoute?: string;
   public ssl: SslConfig;
   public compression: { enabled: boolean; referrerWhitelist?: string[] };
   public csp: ICspConfig;
@@ -171,7 +161,6 @@ export class HttpConfig {
     this.socketTimeout = rawHttpConfig.socketTimeout;
     this.rewriteBasePath = rawHttpConfig.rewriteBasePath;
     this.ssl = new SslConfig(rawHttpConfig.ssl || {});
-    this.defaultRoute = rawHttpConfig.defaultRoute;
     this.compression = rawHttpConfig.compression;
     this.csp = new CspConfig(rawCspConfig);
     this.xsrf = rawHttpConfig.xsrf;
diff --git a/src/core/server/http/http_service.mock.ts b/src/core/server/http/http_service.mock.ts
index 700ae04f00d47..6db1ca80ab437 100644
--- a/src/core/server/http/http_service.mock.ts
+++ b/src/core/server/http/http_service.mock.ts
@@ -68,7 +68,6 @@ const createSetupContractMock = () => {
       getAuthHeaders: jest.fn(),
     },
     isTlsEnabled: false,
-    config: {},
   };
   setupContract.createCookieSessionStorageFactory.mockResolvedValue(
     sessionStorageMock.createFactory()
diff --git a/src/core/server/http/http_service.ts b/src/core/server/http/http_service.ts
index fb12ed80b8e2f..ae9d53f9fd3db 100644
--- a/src/core/server/http/http_service.ts
+++ b/src/core/server/http/http_service.ts
@@ -112,10 +112,6 @@ export class HttpService implements CoreService<InternalHttpServiceSetup, HttpSe
         contextName: T,
         provider: RequestHandlerContextProvider<T>
       ) => this.requestHandlerContext!.registerContext(pluginOpaqueId, contextName, provider),
-
-      config: {
-        defaultRoute: config.defaultRoute,
-      },
     };
 
     return contract;
diff --git a/src/core/server/http/types.ts b/src/core/server/http/types.ts
index 92217515a22a1..9c8bfc073a524 100644
--- a/src/core/server/http/types.ts
+++ b/src/core/server/http/types.ts
@@ -250,16 +250,6 @@ export interface InternalHttpServiceSetup
     contextName: T,
     provider: RequestHandlerContextProvider<T>
   ) => RequestHandlerContextContainer;
-  config: {
-    /**
-     * @internalRemarks
-     * Deprecated part of the server config, provided until
-     * https://github.com/elastic/kibana/issues/40255
-     *
-     * @deprecated
-     * */
-    defaultRoute?: string;
-  };
 }
 
 /** @public */
diff --git a/src/core/server/internal_types.ts b/src/core/server/internal_types.ts
index be4d830c55eab..ff68d1544d119 100644
--- a/src/core/server/internal_types.ts
+++ b/src/core/server/internal_types.ts
@@ -17,7 +17,10 @@
  * under the License.
  */
 
+import { Type } from '@kbn/config-schema';
+
 import { CapabilitiesSetup, CapabilitiesStart } from './capabilities';
+import { ConfigDeprecationProvider } from './config';
 import { ContextSetup } from './context';
 import { InternalElasticsearchServiceSetup } from './elasticsearch';
 import { InternalHttpServiceSetup } from './http';
@@ -47,3 +50,18 @@ export interface InternalCoreStart {
   savedObjects: InternalSavedObjectsServiceStart;
   uiSettings: InternalUiSettingsServiceStart;
 }
+
+/**
+ * @internal
+ */
+export interface ServiceConfigDescriptor<T = any> {
+  path: string;
+  /**
+   * Schema to use to validate the configuration.
+   */
+  schema: Type<T>;
+  /**
+   * Provider for the {@link ConfigDeprecation} to apply to the plugin configuration.
+   */
+  deprecations?: ConfigDeprecationProvider;
+}
diff --git a/src/core/server/legacy/config/__snapshots__/legacy_object_to_config_adapter.test.ts.snap b/src/core/server/legacy/config/__snapshots__/legacy_object_to_config_adapter.test.ts.snap
index ac63f424eabaf..74ecaa9f09c0e 100644
--- a/src/core/server/legacy/config/__snapshots__/legacy_object_to_config_adapter.test.ts.snap
+++ b/src/core/server/legacy/config/__snapshots__/legacy_object_to_config_adapter.test.ts.snap
@@ -11,7 +11,6 @@ Object {
   "customResponseHeaders": Object {
     "custom-header": "custom-value",
   },
-  "defaultRoute": undefined,
   "host": "host",
   "keepaliveTimeout": 5000,
   "maxPayload": 1000,
@@ -43,7 +42,6 @@ Object {
   "customResponseHeaders": Object {
     "custom-header": "custom-value",
   },
-  "defaultRoute": undefined,
   "host": "host",
   "keepaliveTimeout": 5000,
   "maxPayload": 1000,
diff --git a/src/core/server/legacy/config/legacy_object_to_config_adapter.ts b/src/core/server/legacy/config/legacy_object_to_config_adapter.ts
index 397e7a46def58..30bb150e6c15a 100644
--- a/src/core/server/legacy/config/legacy_object_to_config_adapter.ts
+++ b/src/core/server/legacy/config/legacy_object_to_config_adapter.ts
@@ -64,7 +64,6 @@ export class LegacyObjectToConfigAdapter extends ObjectToConfigAdapter {
     return {
       autoListen: configValue.autoListen,
       basePath: configValue.basePath,
-      defaultRoute: configValue.defaultRoute,
       cors: configValue.cors,
       customResponseHeaders: configValue.customResponseHeaders,
       host: configValue.host,
diff --git a/src/core/server/server.ts b/src/core/server/server.ts
index 6ae9fd07f3d5f..4239e7e6c4254 100644
--- a/src/core/server/server.ts
+++ b/src/core/server/server.ts
@@ -257,6 +257,10 @@ export class Server {
     ];
 
     this.configService.addDeprecationProvider(rootConfigPath, coreDeprecationProvider);
+    this.configService.addDeprecationProvider(
+      uiSettingsConfig.path,
+      uiSettingsConfig.deprecations!
+    );
 
     for (const [path, schema] of schemas) {
       await this.configService.setSchema(path, schema);
diff --git a/src/core/server/ui_settings/ui_settings_config.ts b/src/core/server/ui_settings/ui_settings_config.ts
index 702286f953ef1..a54d482a0296a 100644
--- a/src/core/server/ui_settings/ui_settings_config.ts
+++ b/src/core/server/ui_settings/ui_settings_config.ts
@@ -18,15 +18,35 @@
  */
 
 import { schema, TypeOf } from '@kbn/config-schema';
+import { ConfigDeprecationProvider } from 'src/core/server';
+import { ServiceConfigDescriptor } from '../internal_types';
 
-export type UiSettingsConfigType = TypeOf<typeof config.schema>;
+const deprecations: ConfigDeprecationProvider = ({ unused, renameFromRoot }) => [
+  unused('enabled'),
+  renameFromRoot('server.defaultRoute', 'uiSettings.overrides.defaultRoute'),
+];
 
-export const config = {
+const configSchema = schema.object({
+  overrides: schema.object(
+    {
+      defaultRoute: schema.maybe(
+        schema.string({
+          validate(value) {
+            if (!value.startsWith('/')) {
+              return 'must start with a slash';
+            }
+          },
+        })
+      ),
+    },
+    { allowUnknowns: true }
+  ),
+});
+
+export type UiSettingsConfigType = TypeOf<typeof configSchema>;
+
+export const config: ServiceConfigDescriptor<UiSettingsConfigType> = {
   path: 'uiSettings',
-  schema: schema.object({
-    overrides: schema.object({}, { allowUnknowns: true }),
-    // Deprecation is implemented in LP.
-    // We define schema here not to fail on the validation step.
-    enabled: schema.maybe(schema.boolean()),
-  }),
+  schema: configSchema,
+  deprecations,
 };
diff --git a/src/core/server/ui_settings/ui_settings_service.test.ts b/src/core/server/ui_settings/ui_settings_service.test.ts
index d908a91a39c70..9b6a5ba0a4884 100644
--- a/src/core/server/ui_settings/ui_settings_service.test.ts
+++ b/src/core/server/ui_settings/ui_settings_service.test.ts
@@ -22,7 +22,6 @@ import { MockUiSettingsClientConstructor } from './ui_settings_service.test.mock
 
 import { UiSettingsService } from './ui_settings_service';
 import { httpServiceMock } from '../http/http_service.mock';
-import { loggingServiceMock } from '../logging/logging_service.mock';
 import { savedObjectsClientMock } from '../mocks';
 import { mockCoreContext } from '../core_context.mock';
 
@@ -69,27 +68,6 @@ describe('uiSettings', () => {
         expect(MockUiSettingsClientConstructor.mock.calls[0][0].overrides).toEqual(overrides);
       });
 
-      it('passes overrides with deprecated "server.defaultRoute"', async () => {
-        const service = new UiSettingsService(coreContext);
-        const httpSetupWithDefaultRoute = httpServiceMock.createSetupContract();
-        httpSetupWithDefaultRoute.config.defaultRoute = '/defaultRoute';
-        const setup = await service.setup({ http: httpSetupWithDefaultRoute });
-        setup.asScopedToClient(savedObjectsClient);
-
-        expect(MockUiSettingsClientConstructor.mock.calls[0][0].overrides).toEqual({
-          ...overrides,
-          defaultRoute: '/defaultRoute',
-        });
-
-        expect(loggingServiceMock.collect(coreContext.logger).warn).toMatchInlineSnapshot(`
-          Array [
-            Array [
-              "Config key \\"server.defaultRoute\\" is deprecated. It has been replaced with \\"uiSettings.overrides.defaultRoute\\"",
-            ],
-          ]
-        `);
-      });
-
       it('passes a copy of set defaults to UiSettingsClient', async () => {
         const service = new UiSettingsService(coreContext);
         const setup = await service.setup(setupDeps);
diff --git a/src/core/server/ui_settings/ui_settings_service.ts b/src/core/server/ui_settings/ui_settings_service.ts
index db08c3cad85a2..942c2625ac8e7 100644
--- a/src/core/server/ui_settings/ui_settings_service.ts
+++ b/src/core/server/ui_settings/ui_settings_service.ts
@@ -56,7 +56,9 @@ export class UiSettingsService
   public async setup(deps: SetupDeps): Promise<InternalUiSettingsServiceSetup> {
     registerRoutes(deps.http.createRouter(''));
     this.log.debug('Setting up ui settings service');
-    this.overrides = await this.getOverrides(deps);
+    const config = await this.config$.pipe(first()).toPromise();
+    this.overrides = config.overrides;
+
     return {
       register: this.register.bind(this),
       asScopedToClient: this.getScopedClientFactory(),
@@ -95,18 +97,4 @@ export class UiSettingsService
       this.uiSettingsDefaults.set(key, value);
     });
   }
-
-  private async getOverrides(deps: SetupDeps) {
-    const config = await this.config$.pipe(first()).toPromise();
-    const overrides: Record<string, any> = config.overrides;
-    // manually implemented deprecation until New platform Config service
-    // supports them https://github.com/elastic/kibana/issues/40255
-    if (typeof deps.http.config.defaultRoute !== 'undefined') {
-      overrides.defaultRoute = deps.http.config.defaultRoute;
-      this.log.warn(
-        'Config key "server.defaultRoute" is deprecated. It has been replaced with "uiSettings.overrides.defaultRoute"'
-      );
-    }
-    return overrides;
-  }
 }
diff --git a/src/legacy/server/config/schema.js b/src/legacy/server/config/schema.js
index 64e778642aad9..ac490e370102f 100644
--- a/src/legacy/server/config/schema.js
+++ b/src/legacy/server/config/schema.js
@@ -70,7 +70,6 @@ export default () =>
 
     server: Joi.object({
       name: Joi.string().default(os.hostname()),
-      defaultRoute: Joi.string().regex(/^\//, `start with a slash`),
       // keep them for BWC, remove when not used in Legacy.
       // validation should be in sync with one in New platform.
       // https://github.com/elastic/kibana/blob/master/src/core/server/http/http_config.ts
diff --git a/src/legacy/server/http/setup_default_route_provider.ts b/src/legacy/server/http/setup_default_route_provider.ts
index 0e7bcf1f56f6f..9a580dd1c59bd 100644
--- a/src/legacy/server/http/setup_default_route_provider.ts
+++ b/src/legacy/server/http/setup_default_route_provider.ts
@@ -29,7 +29,7 @@ export function setupDefaultRouteProvider(server: Legacy.Server) {
 
     const uiSettings = request.getUiSettingsService();
 
-    const defaultRoute = await uiSettings.get('defaultRoute');
+    const defaultRoute = await uiSettings.get<string>('defaultRoute');
     const qualifiedDefaultRoute = `${request.getBasePath()}${defaultRoute}`;
 
     if (isRelativePath(qualifiedDefaultRoute, serverBasePath)) {