All queries for Group entities, GroupContent entities and entities that are added to a group using a plugin with 'entity_access' set to TRUE will now properly check access.
Queries in code
This means that any code you have that runs an entity query and does not care about access will now need to set ->accessCheck(FALSE) if it still wants to retrieve all IDs regardless of access. If you do care about only showing the IDs the user has access to, that is now taken care of automatically!
$child_count = $this->entityTypeManager
->getStorage('group')
->getQuery()
->condition('type', $child)
// WITHOUT THIS LINE, YOU WILL ONLY GET GROUP IDS THE USER HAS ACCESS TO
->accessCheck(FALSE)
->count()
->execute();
Views
For Views, this also applies but only for the table you're basing your View on. So if you start building your View with the Node entity type, it will check for node access. If you start from GroupContent, it will only check for GroupContent access, etc. You can turn this off by using the "disable query rewriting" flag in the View's advanced options.
Your custom plugins
If you have any custom GroupContentEnabler plugin, it needs to specify a "permission_provider" and "access" handler for query access to work. Please look at GroupNode or GroupMembership for examples.
