Every time I see a question asking to verify a proof (usually some exercise), the tag is just not there. I often edit the question to add it but... does it really make sense to keep it?
Since every proof is about some topic in maths, this topic would already be covered by another tag. I can't think of a question who could have the unique tag "proof-verification".
Moreover, who is ever going to search questions by the "proof-verification" tag?
I really don't see its usefulness. I understand it could be a way for some users to blacklist those questions, but in my experience almost nobody ever uses that tag.