Since T166854, we have ContentTranslationUserGroupTargetNamespace configuration option to set default namespace per user group level. That config is not currently used anywhere. Not sure if any wiki was configured to use it at some point, but looks unused since introduced, two year ago.
In T217237, enwiki was configured to restrict users outside "extendedconfirmed" group from publishing to main namespace. After that, all restricted users see error from T192144, because main namespace is the default for them. That error has action button that allows them to change to user namespace, and they can switch using publish settings anytime as well.
But, we could reverse this process and make user go through less steps by utilizing target namespace per user group config, so users outside "extendedconfirmed" group have user namespace by default and get error only if they change to main namespace.
Config would look something like:
'wgContentTranslationUserGroupTargetNamespace' => [ 'default' => , 'enwiki' => [ 'extendedconfirmed' => 0, '*' => 2, ], ],
If we don't want to make this change for enwiki or other wikis with similar restrictions (plwiki, kkwiki, nowiki), removing the config option and associated code should be the way to go.