I guess that Google removes already indexed pages that are now returning the HTTP error
Unfortunately I can't find any definitive statement on that matter. Do you know the answer?
They get removed from the index. There is no official documentation from Google on this, but you can find plenty of cases where websites have been returning 403 by mistake and got deindexed.
As you know Google will eventually remove pages which return server codes
Google will most likely continue checking the page and keep it in it's index in the event it becomes accessible. If you would like it to not be indexed set the meta noindex header tag