Expanding my comments & thoughts into an answer:
From the sounds of the discussions playing out here, I don't think we have anything against 'Longplaying' games. Longplaying is a form of gameplay after all, and we are a gaming Q&A resource, so questions which pertain to gameplay in the context of performing a Longplay (or otherwise) are on topic for us.*
How do we use tags?
Tags exist primarily to categorise content. Every community handles this differently, but I know of two main ways this is handled (on SE sites at least): 'Overarching' tagging and 'Specific' tagging
'Overarching' tagging uses larger-topic-area tags as a baseline, and applies more specific tags where they make the most sense. If this is how we worked on Arqade, it would look something like this:
This may make more sense on sites like StackOverflow - a question about C# 4.0 is still a question about C#, but it doesn't generally make much sense here as our 'topics' are generally smaller - someone who is playing Sonic 2 isn't suddenly an expert on Sega, or the Genesis, or even other Sonic games. So instead we typically stick to the smallest specific tags that can apply: individual tags based on what the question is actually about. 90% of the time this is a game or console title. For example:
There are some exceptions of course where 'overarching' tags make sense for us:
There are some others but these would be the ones most people are familiar with. These tags may span multiple games, communities or consoles, and are generally well received for the functionality and groupings they provide. Likewise, our larger topics can have sub-tags as well: minecraft-commands as the prime example, but also DLCs like skyrim-dawnguard.
So what about [Longplay]?
We've talked about overarching vs specific tagging, and some exceptions where the usefulness of the tag outweighs the awkwardness of applying overarching tags. So where does longplay lie?
1. Does it provide a useful 'overarching' grouping of like-minded content?
My answer is No - Like Vemonus states in the question:
We have plenty of questions asking how to unlock easter eggs or alternate endings, which are not necessarily related to longplays.
Questions about game functionality as they pertain to longplays are no different than just questions about game functionality. So the longplay tag shouldn't be used on questions about game strategies.
2. Is the only (current) question a valid use of the tag?
I would say Yes, it's certainly about Longplays in general, rather than something game-specific. Could the question exist without the tag? Probably, it could live under terminology as it is concerned with how the longplay community name/sort their playthroughs. But why would we remove it when it adequately describes the question?
3. Do we keep tags with only one question?
We do!. Due to the nature of games and their popularity, plenty of games/consoles etc may only have a few questions that are useful and valid.
4. Can [longplay] be useful as a 'specific' topic area?
The big question. Assuming the "Tag is for questions about Tag and Tag only" logic, can we use the tag for questions about Longplays in general? Can questions be asked about Longplay rules and regulations that don't pertain to a particular game?
I say Maybe. Not very committal I know, but I find it hard to pass judgement on a tag that has literally just been created.
I think we should monitor it's use (if any - see point #2 again) and act when we see a problem, rather than preemptively trying to shut it down based on what might happen.
To that end, I've added a Tag Wiki excerpt to the longplay tag which describes this:
a playthrough of a game for documenative or showcase reasons. Use this tag for questions regarding longplay formats, rules, or non-game specific conventions. For questions about a game in the context of performing a longplay, please use the game's tag instead.
I think the tag should stay, for now. It adequately categorises it's one question and hasn't posed a problem for us yet.
* Other rules still apply and close reasons like 'Too Broad' or 'Primarily Opinion Based' can be applied as needed on a question-by-question basis