I've noted myself and other users have also seemed to note that stories can no longer be tagged with the "7" tag. Did I miss a memo somewhere?
I tried editing a question with that tag, and I got an error about that tag not being allowed. I will check what happened, but it seems the software running Stack Exchange doesn't allow that tag in the questions. I have a vague idea of why this happens; I just need to verify it.
As per Are tags with a name made of just numbers still allowed? this has been now fixed. If they change the code, and introduce back this behavior, they will also make a post on the main meta site to explain why they introduced back this limitation.
Sorry about this; this was my bad. It was impacting an API. I verified that we didn't have pure numeric tags on some number of sites, but I obviously failed and didn't thoroughly check all of them, which is my error. I will find another approach for the problem that I was trying to fix. The code has already been reverted. Again, sorry.