Can you objectively determine whether the tag does or does not apply to a given question?
Kinda. Any question asking for any kind of server or server-like behaviour should go with this tag.
Can you say that a question is “about ”?
Yes and not. I know what a server is "to server some resource" but after that I have no idea what else I could tell about it.
I think that we should stress the rule:
(...) tags that cannot stand alone as the only tag on a question, are not allowed.
A question using server as single tag is not good enough to tell me something about the question except a vague idea that OP is asking for some resource serving application. So, no, we should remove the server tag.