Add release note for mdn#9400

RELEASE_NOTES.md

@@ -8,6 +8,10 @@ March 11, 2021
 
 - [Support statement objects](https://github.com/mdn/browser-compat-data/blob/v3.2.0/schemas/compat-data-schema.md#the-support_statement-object) may no longer use `false` or `null` values for `version_removed`. Values for `version_removed` are now required to be real version numbers or `true` (removed in some unknown version). This change to the schema codifies a widely-followed practice within existing compatability data. ([#9015](https://github.com/mdn/browser-compat-data/pull/9015))
 - `api.Window.OverconstrainedError` was removed as a duplicate of `api.OverconstrainedError`. ([#9401](https://github.com/mdn/browser-compat-data/pull/9401))
+- The following no-longer supported features have been removed as irrelevant:
+  - `api.PasswordCredential.additionalData` ([#9400](https://github.com/mdn/browser-compat-data/pull/9400))
+  - `api.PasswordCredential.idName` ([#9400](https://github.com/mdn/browser-compat-data/pull/9400))
+  - `api.PasswordCredential.passwordName` ([#9400](https://github.com/mdn/browser-compat-data/pull/9400))
 - `api.SVGFECompositeElement.in1` replaces `api.SVGFECompositeElement.in`, fixing a misidentified attribute (again). ([#9074](https://github.com/mdn/browser-compat-data/pull/9074), [#9343](https://github.com/mdn/browser-compat-data/pull/9343))
 - The following never-implemented features were removed:
   - `api.PaymentManager.requestPermission` ([#9399](https://github.com/mdn/browser-compat-data/pull/9399))