Add release note for mdn#9830

RELEASE_NOTES.md

@@ -10,6 +10,10 @@ August 19, 2021
 
 ### Notable changes
 
+- The XPath (`xpath`) and XSLT (`xslt`) features have been removed. These features poorly covered their subjects and were unmaintained. Rather than purport to have such data, we removed it.
+
+  We're not ruling out the return of XPath and XSLT data in the future. But for now, in the absence of an intentional effort to create and maintain that data, we thought it best to remove the faulty data. ([#9830](https://github.com/mdn/browser-compat-data/pull/9830))
+
 - The schema for browser release data now supports the optional `accepts_flags` boolean hint, which indicates whether a browser (or individual release) permits user-configurable preferences. Presently, this is a hint for data contributors or tools. The value of `accepts_flags` doesn't make any guarantees about the contents of feature data. ([#11286](https://github.com/mdn/browser-compat-data/pull/11286))
 
 - Descendant features of `api.HTMLCanvasElement.getContext` and `api.HTMLCanvasElement.toBlob` have been reorganized to follow [the guideline for parameter object features](https://github.com/mdn/browser-compat-data/blob/main/docs/data-guidelines.md#parameters-and-parameter-object-features). See [the PR for a detailed list of features affected](https://github.com/mdn/browser-compat-data/pull/11234#issuecomment-900265901). ([#11234](https://github.com/mdn/browser-compat-data/pull/11234))