In the get_contexts_for_userid() provider method, it would be useful to be able to add known contexts directly when a plugin stores all its data at that context. The only two use cases for this I can think of are the system context and the user context, so it may not be worth making set_contextids() public, but a couple of helper methods like add_system_context() and add_user_context($userid) would be useful.

At the moment you have to do something like this, which seems absurd:

    public static function get_contexts_for_userid(int $userid): contextlist {

        $result = new contextlist('local_someplugin');

        $result->add_from_sql('SELECT id FROM {context} WHERE id = ?', [\context_system::instance()->id]);

        return $result;

    }