There's a ping group tag that's taken and that I'm interested in, but it was created by a user who was last online more than 4 years ago. So I guess my question is.. are there any plans to ever make unused ping group tags available again?

not that us regular users know. I haven't even seen it come up because it's easy to make a new one, just add another word or number at the end.

You might try to make a thread in Feedback or Suggestions about it.