Add release notes for slim sidebar

docs/releases/2.16.md

@@ -8,6 +8,9 @@
 
 ## What's new
 
+### Slim sidebar
+
+As part of a [wider redesign](https://github.com/wagtail/wagtail/discussions/7739) of Wagtail’s administration interface, we have replaced the sidebar with a slim, keyboard-friendly version. This re-implementation comes with significant accessibility improvements for keyboard and screen reader users, and will enable us to make navigation between views much snappier in the future. Please have a look at [upgrade considerations](#upgrade-considerations) for more details on differences with the previous version.
 
 ### Other features
 
@@ -91,3 +94,13 @@ The internal (undocumented) class-based view `wagtail.admin.views.generic.Delete
 ### Renamed admin/expanding-formset.js
 
 `admin/expanding_formset.js` has been renamed to `admin/expanding-formset.js` as part of frontend code clean up work. Check for any customised admin views that are extending expanding formsets, or have overridden template and copied the previous file name used in an import as these may need updating.
+
+### Deprecated sidebar capabilities
+
+The new sidebar largely supports the same customisations as its predecessor, with a few exceptions:
+
+- Top-level menu items should now always provide an `icon_name`, so they can be visually distinguished when the sidebar is collapsed.
+- `MenuItem` and its sub-classes no longer supports customizing arbitrary HTML attributes.
+- `MenuItem` can no longer be sub-classed to customise its HTML output or load additional JavaScript
+
+For sites relying on those capabilities, we provide a [`WAGTAIL_SLIM_SIDEBAR = False`](../reference/settings/#WAGTAIL_SLIM_SIDEBAR) setting to switch back to the legacy sidebar. The legacy sidebar and this setting will be removed in Wagtail 2.18.