could now include the Elementary OS site.
Note: This answer now also exists as separate feature request here on meta.askubuntu.com.
Further updates and discussion should take place over there.
If you like this answer, please make sure you also upvote the linked question. Thanks!
What about rewording the banner to something like
This is not about Ubuntu.
Please visit our [off-topic compass] to find out on which partner site in the Stack Exchange network your problem might fit best.
And then create a nice community-wiki site here on meta.askubuntu.com which looks like this:
Q: My question was marked as off-topic, where should I ask it instead?
You posted a question here and excitedly waited for an answer, but received a close banner like this?
Read through the answers below to find out where your question would fit best!
A: Unix & Linux
Every question about any UNIX or Linux based system is allowed here.
However, if you use for example Ubuntu, ElementaryOS, OSX or Android, there are specialized sites for them, where you can get more specific answers.
A: Stack Overflow
All general programming questions should be asked here.
If you have working code and just look for optimizations, you're better of at Code Review though.
A: Super User
This is the most general site all about computers and related stuff. Every operating system is welcome, and so are hardware problems.
... and so on, one answer for each possibly related site.
The off-topic banner isn't meant to be all encompassing. That would be impossible to maintain and extremely messy. It covers the basic and most common occurrences, it's up to the user to figure the rest out. That's not our job. What do you propose we do if/when sites specific to Mint, Kali, or [insert other Ubuntu based distribution here] launch?
As it is we only have 28 characters remaining in the existing reason anyway, so we couldn't add it even if we wanted to.
Sorry, the message is space limited and we need to mention Windows, Mac and just don't have the room (or in my case, inclination) to list every non-Ubuntu distribution directly.