Here on AskUbuntuAsk Ubuntu we have quite a lot of tags for various Linux distributions/Operating Systems/things that could be off-topic if specific about them:
debian, mint, fedora, crunchbang, windows, android, macosx
People do tag questions which aren't about Ubuntu but are about this tag - so it could be a good idea to have a warning that could appear next to the question - something like:
N.B. This shouldn't prevent a user from posting the question, just delay them a bit.
It could also have bits like 'you may want to post the question here' as well, where here is a relevant SE site, and a link to the on/off-topic page in the help center - even 'please include your version of Ubuntu' etc
I know the tag descriptions already have indications about this:
But something like this would be more attention grabbing, particularly for those users that just add tags because they are prompted to, and completely ignore the tag descriptions:
This rough idea could possibly be applied elsewhere - e.g. questions about EOL releases