Generally just because another site exists that covers a subset of Super User doesn't make all of that subset found on Super User no longer welcome or wanted.
Related to migrations, most are not migrated unless the asker themselves prefer to shift the target audience.
To clear up any implicit "go somewhere else instead and don't bother here" tone, it's been reworded to:
Free, open-source operating system based on Debian GNU/Linux. Ubuntu questions are on topic here, but if you prefer an Ubuntu-only audience there is AskUbuntu.com.
With the tag info page expanding on it slightly with:
The related Stack Exchange site, AskUbuntu, exists if you would prefer an Ubuntu-only audience, as opposed to the multi-environment/operating system one found here on Super User.
Which should better reason why a user may want to consider the other site.