I'd say it's...difficult.
It's not as if there's no value in properly analysing a fan theory for its plausibility, and I certainly wouldn't call that off-topic per se. It's very much about the film's story and its implications. For example we also (more or less successfully) discussed a famous fan-theory about another popular universe. But there can be problems, especially when the story hasn't actually ended yet and it just seems to be speculation about how it might end.
You are certainly right in fearing the speculative nature of these questions a little. The problem is that theories like that usually spring from some kind of informational vacuum. There is not enough explanation in the films, so people fill the gaps with what they think makes sense. But the fact that there's this vacuum in the first place also makes it difficult to disprove these theories, especially since often enough they come from people who have seen every single MCU film (and likely read some comics, too) so there either is not enough information in the films to disprove it, or they chose to interpret that information to fit their theory. So these things can get quite opinionated and speculative. (And if it is easily disprovable by just referencing a particular scene, then the value of the question is debatable, too.)
Another aspect is that often enough, especially in the case of Avengers: Infinity War and its huge cliffhanger, they also come from people just being impatient for how the story ends in the next film, so they make up all kinds of things for how it could end. But for all we know the next film might make their theory entirely impossible, and thus also void any question about it of its very premise.
Because of the above reasons, and a ton of "what if..." and "why didn't..." questions we already have about Infinity War, it seems the community is also a little tired of these questions. Now this doesn't mean they're necessarily off-topic, but it's a factor to be considered when anticipating the questions' possible reception.
So on the bottom line, I wouldn't call a blanket stop to questions debating (somewhat significant and reasonable) fan theories, but I'd be really careful with them and it depends on the specific case a little. Don't be too surprised or upset if they end up closed or poorly received or require a little more effort to make them going. But if you really achieve to get the question going, we're happy to have an interesting plot analysis contributing to the films' appreciation.
