It is true that questions like the おやすみNASA胃 one can be extremely "local" and not that useful for future readers... although, even questions like that could be useful to others in the future if someone happens to answer them with detailed info on ways that you can play with writing systems in Japanese, say.
But I can also imagine questions about puns that are useful in general because they refer to well-known "standard" puns (even if deprecated now), not one-offs. For example, I think a question like "I saw someone say おいP instead of おいしい, what's that all about?" is fine. (Does おいP count as a pun? Can we say that it does for the purposes of my example?) おいP is quite obscure, but it's not a one-off -- it's (most likely) a reference to a weird corner of the language/culture which the passage of time has made less accessible, and I can imagine multiple people running across it on the internet and wondering what it is.
I guess the downside of this is that you can't tell whether a given pun is a one-off or not until someone answers the question about it, but individual questions can still be downvoted, and it doesn't look like the tag is being abused so far so I don't see much reason to declare it invalid.