The hash idea works even in mutable land. There are obvious caveats to not mutating data in race related scenarios. But often snapshots and other "freeze" based ideas are just as workable as moving entirely to alternatives.
Uh, how? Other than removing it from the dictionary, mutating it then adding it back.
I fully grant that immutable structures take away the concerns over data being coherent together. Such that it's easy to do without worrying about someone changing the data under you.
Oddly, it can sometimes make it even harder to know that data is stale. Since updating a small part can devolve into a chore. Still fully agreed that that is the best default position.