Does the plaster have to go?

[Jan-Å]

I would skip the plastering anyway and shim out for the studs where needed, it doesn't matter if there's a little air behind in some places, right?

But you seem quite eager to strip the plaster so try a little where it's already bad to see how much comes off?
If it's easy and a lot falls off by itself, just continue; otherwise, you can skip it.