Add note about extending section not subject to deprecation policy

docs/contributing/release_process.md

@@ -74,9 +74,12 @@ that it's time to remove them.
 Wagtail uses a loose form of [semantic versioning](https://semver.org/).
 SemVer makes it easier to see at a glance how compatible releases are with each
 other. It also helps to anticipate when compatibility shims will be removed.
+
 It's not a pure form of SemVer as each feature release will continue to have a
 few documented backwards incompatibilities where a deprecation path isn't
-possible or not worth the cost.
+possible or not worth the cost. This is especially true for features documented
+under the [](../extending/index) section of the documentation and their
+corresponding API reference, which tend to be more actively developed.
 
 We try to strike the balance between:
 

docs/extending/index.md

@@ -4,6 +4,13 @@ The Wagtail admin interface is a suite of Django apps, and so the familiar conce
 
 This section describes the various mechanisms that can be used to integrate your own code into Wagtail's admin interface.
 
+```{note}
+The features described in this section and their corresponding reference
+documentation are not subject to the same level of stability described in our
+[](deprecation_policy). Any backwards-incompatible changes to these features
+will be called out in the upgrade considerations of the [](../releases/index).
+```
+
 ```{toctree}
 ---
 maxdepth: 2