I propose that new users who recently ran into a ban from asking questions anywhere else on the network be shown a warning before asking a question here on Meta Stack Exchange.
A huge portion of those questions are either users asking about their ban (who should ask on their per-site meta instead) or asking their low-quality question here and trying to get an answer here. There's a very small chance of it being a legitimate case where a new user wants to post a question here on Meta Stack Exchange if they're question-banned on any other site in the network. The majority of the time, posts from new, banned users are either blatantly off-topic, off-topic because they're about one specific site, or a duplicate of What can I do when getting "We are no longer accepting questions/answers from this account"?.
A warning telling users that they probably shouldn't be posting here would likely reduce the total number of off-topic questions asked here (both blatantly off-topic and site-specific).
An example of such a notice would be:
We noticed that you're not currently able to ask questions on [site]. Please read the following information carefully before posting here:
If you wish to ask about your ban: Please carefully read our guidance for banned users. If you have a further question that was not answered in that post, please post your question on the per-site meta of the site you're banned on.
Please make sure your question is on-topic for this site. This site, Meta Stack Exchange, is not the right place to post a question that you intended to post on a different site because you're banned from that site. If you post your question here, it will be closed as off-topic.
Update: As per gnat's comment, it's possible to tell whether or not a user has attempted to ask a question once a ban is in place. As such, I'm changing this request to something more technically feasible.