I just found out there's a ubuntu-17.10 tag with two questions currently.
We don't seem to have tags like that for other Ubuntu release numbers. No ubuntu-16.04, for example, just 16.04. Since there were only two questions tagged ubuntu-17.10, I've just retagged them to 17.10.
The system will remove the tag after a while if it is not applied to any more questions. If it comes back and people start using it on multiple posts, I'd definitely support making it a synonym, but I somewhat doubt that will happen.