A \Joomla\Database\DatabaseInterface is supposed to be injected into an \Joomla\CMS\MVC\Factory\MVCFactory. In case that would be forgotten, for instance when instantiating your own MVCFactory without making use of the \Joomla\CMS\Extension\Service\Provider\MVCFactory service provider, as a temprorary band-aid, the database is fetched from the container.

In https://github.com/joomla/joomla-cms/blob/5.0-dev/libraries/src/MVC/Factory/MVCFactory.php#L174-L175 a "deprecated" error-message is (suppressed) triggered and, as a temprorary band-aid, the database that is missing in the MVCFactory to inject in a DatabaseAwareInterface MVC object, is fetched from the container in a catch block:

    @trigger_error(sprintf('Database must be set, this will not be caught anymore in 5.0.'), E_USER_DEPRECATED);
    $model->setDatabase(Factory::getContainer()->get(DatabaseInterface::class));

The same for a MVC-Table object in lines 270-271.

But this code is still present in Joomla 5.

Should we now remove the band-aid in line 175 (and 271) and let it (unsuppressed) completely fail on line 174 ( and 270) in Joomla 5.0.2, or postpone that to 5.1.0 or even to 6.0.0? For there can be some 3PD code (I've lately seen one) that would fail on a Joomla-update if we correct this now.