since listing a decorator claims that it's safe, we should believe the user.

As it happens I just found a case where the transform decorator in python-trio/trio#638 isn't fully safe (it can hang indefinitely on early exit)... I think the solution to that is to upstream a working version into Trio rather than continuing to have them proliferate with bugs, and so we'll presumably expand the default allowlist with that once (if?) my upcoming PR is merged.