problem-domain is used in a couple of ways - but it seems it's just another meta-tag. Should it be removed from the relevant questions, of which there are 10 at the moment of writing?
It should definitely go.
I've voted to close most of them, and am removing the tag from the remainder. There are now 5 left over, let's see if we can get rid of those (close, then delete).
Current status: We are down to 3 questions: