DOC Document change to return types in composer plugins (#685)

en/08_Changelogs/5.4.0.md

@@ -146,6 +146,18 @@ See [indexes](/developer_guides/model/indexes/) to learn about indexes in Silver
 
 ## API changes
 
+### Return type changes in composer plugins
+
+We have made an exception to our [definition of public API](/project_governance/public_api/) by adding a return type to the `execute()` method in the symfony commands for `silverstripe/vendor-plugin` and `silverstripe/recipe-plugin`.
+
+The exception was made for the following reasons:
+
+- This change ensures CMS 5 can continue to be installed in scenarios where Composer is installed with Symfony 7. This can be done manually, and is also likely to be a future default.
+- The likelihood of someone subclassing the relevant classes is very low.
+- For installations of installer with earlier versions of Symfony this won't cause any problems, since PHP allows return types to be more specific in subclasses. This is known as [covariance](https://www.php.net/manual/en/language.oop5.variance.php#language.oop5.variance.covariance).
+
+### Deprecated API
+
 - The class names for the `TopPage` feature in [`dnadesign/silverstripe-elemental`](https://github.com/silverstripe/silverstripe-elemental) do not follow the correct naming convention for Silverstripe CMS. The existing classes have been deprecated and will be renamed to match the correct naming convention in a future major release.
 - [`SilverStripe\ORM\ArrayLib`](api:SilverStripe\ORM\ArrayLib) has been deprecated. It will be renamed to `SilverStripe\Core\ArrayLib`
 - [`SilverStripe\ORM\ArrayList`](api:SilverStripe\ORM\ArrayList) has been deprecated. It will be renamed to `SilverStripe\Model\List\ArrayList`