Questions about physics are not closed based on topic here. They are closed based on policy.
For example, I could ask a question about projectile motion that would be closed if it is
- a homework question asking how to do a calculation rather than a conceptual question about the physics of projectile motion
- a question asking how to design a device to track a projectile through the air
- a question just asking how to solve for $t$ in $y=v_0t+\frac12at^2$
- a vague question where it is not clear what is being asked of, such as "how often does projectile motion succeed on Earth?"
- a post that involves multiple questions about projectile motion all at once
- a question asking for opinions, such as "is projectile motion your favorite type of motion, and if not what is?", or "why is projectile motion taught to introductory physics students so often?"
Any of the closure reasons above are valid, but that wouldn't mean projectile motion is a forbidden subject here. The only subjects that are not allowed here are ones that are not about physics at all. But if you pick anything related to physics I am sure you could make a question about that topic that would remain open here (I'm sure there are edge cases, but I think you can get the point I'm making here).
Furthermore, a question being closed for whatever reason doesn't mean we think that subject is equivalent to the closure reason. For example, we don't think all projectile motion questions are off-topic homework questions.
Therefore, in your case, just because your question was closed does not mean as non-mainstream doesn't mean MWI is considered to be non-mainstream or isn't allowed on this site in general. I am sure you can find MWI questions here that were not closed.