I think the dialogue when you try to leave a project that is locked could be a little better maybe? Right now it just says "You can't leave" and "The membership is locked" more or less; should I surface a link to the policy stuff there too?

Oh, I think this is fine as-is. Since this is basically an administrative option, hopefully the reasoning will be clear from context. I think we can wait for users to get confused, at least.

If they do get confused, I think they're probably most likely to be confused about why a project is locked, which we can't easily answer anyway. Like, the information that's most helpful would be "who locked it (which we do know) and why (which we don't)", not "who is allowed to lock projects". So we could add some kind of "lock reason" prompt or something, but this seems like overkill unless users are actually confused.