Deprecate compatibility mode settings

This includes adding a private setting: `$_govuk-is-legacy-review-app` which is set to `true` in the legacy app stylesheets within the govuk-frontend local review app. We create this temporary dependancy so that warnings aren't fired locally but are fired for users where compatibility mode settings are true in their own sass.

CHANGELOG.md

@@ -86,6 +86,16 @@ This was added in [pull request #2677: Amend error summary markup to fix page lo
 
 ### Deprecated features
 
+#### Stop using the compatibility mode settings
+
+In GOV.UK Frontend v5.0 we will stop supporting compatibility with legacy codebases. We are therefore deprecating the compatibility mode variables associated with legacy codebases:
+
+- `$govuk-compatibility-govukfrontendtoolkit`
+- `$govuk-compatibility-govuktemplate`
+- `$govuk-compatibility-govukelements`
+
+This was introduced in [pull request #2882: Deprecate compatibility mode settings](https://github.com/alphagov/govuk-frontend/pull/2882).
+
 #### Stop using settings associated with legacy codebases
 
 In GOV.UK Frontend v5.0 we will stop supporting compatibility with legacy codebases. As part of this, we are deprecating settings controlled by compatibility mode variables. This includes the `govuk-compatibility` mixin and the following settings:
@@ -95,7 +105,7 @@ In GOV.UK Frontend v5.0 we will stop supporting compatibility with legacy codeba
 - `$govuk-typography-use-rem`
 - `$govuk-font-family-tabular`
 
-This was introduced in [pull request #2844: Deprecate compatibility mode settings](https://github.com/alphagov/govuk-frontend/pull/2844).
+This was introduced in [pull request #2844: Remove compatibility mode from govuk-frontend](https://github.com/alphagov/govuk-frontend/pull/2844).
 
 ### Fixes
 

app/assets/scss/app-legacy-ie8.scss

@@ -8,6 +8,11 @@ $govuk-compatibility-govukfrontendtoolkit: true;
 $govuk-compatibility-govuktemplate: true;
 $govuk-compatibility-govukelements: true;
 
+// Suppress compatibility mode deprecation warnings locally
+$govuk-suppressed-warnings: (
+  "compatibility-mode"
+);
+
 // Set Elements assets path
 $path: "/vendor/govuk_frontend_toolkit/assets/";
 

app/assets/scss/app-legacy.scss

@@ -8,6 +8,11 @@ $govuk-compatibility-govukfrontendtoolkit: true;
 $govuk-compatibility-govuktemplate: true;
 $govuk-compatibility-govukelements: true;
 
+// Suppress compatibility mode deprecation warnings locally
+$govuk-suppressed-warnings: (
+  "compatibility-mode"
+);
+
 // Set Elements assets path
 $path: "/vendor/govuk_frontend_toolkit/assets/";
 

src/govuk/settings/_compatibility.scss

@@ -23,9 +23,17 @@
 ///
 /// @type Boolean
 /// @access public
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $govuk-compatibility-govukfrontendtoolkit: false !default;
 
+@if $govuk-compatibility-govukfrontendtoolkit == true {
+  @include _warning("compatibility-mode", "$govuk-compatibility-govukfrontendtoolkit " +
+  "is deprecated. From version 5.0, GOV.UK Frontend will not support compatibility " +
+  "with the legacy codebase govuk_frontend_toolkit");
+}
+
 /// Compatibility Mode: alphagov/govuk_template
 ///
 /// Enabling this will:
@@ -41,9 +49,17 @@ $govuk-compatibility-govukfrontendtoolkit: false !default;
 ///
 /// @type Boolean
 /// @access public
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $govuk-compatibility-govuktemplate: false !default;
 
+@if $govuk-compatibility-govuktemplate == true {
+  @include _warning("compatibility-mode", "$govuk-compatibility-govuktemplate " +
+  "is deprecated. From version 5.0, GOV.UK Frontend will not support " +
+  "compatibility with the legacy codebase govuk_template");
+}
+
 /// Compatibility Mode: alphagov/govuk_elements
 ///
 /// Enabling this will:
@@ -56,16 +72,26 @@ $govuk-compatibility-govuktemplate: false !default;
 ///
 /// @type Boolean
 /// @access public
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $govuk-compatibility-govukelements: false !default;
 
+@if $govuk-compatibility-govukelements == true {
+  @include _warning("compatibility-mode", "$govuk-compatibility-govukelements " +
+  "is deprecated. From version 5.0, GOV.UK Frontend will not support compatibility " +
+  "with the legacy codebase govuk_elements");
+}
+
 /// Compatibility Product Map
 ///
 /// Maps product names to their settings that we can use to lookup states from
 /// within the `@govuk-compatibility` mixin.
 ///
 /// @type Map
 /// @access private
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $_govuk-compatibility: (
   govuk_frontend_toolkit: $govuk-compatibility-govukfrontendtoolkit,