This discussion belongs on a different thread rather than resurrecting an old one that lots of people are following.
For the avoidance of doubt: there is no ambiguity here. Underscored attributes are not source stable and that means you need to deal with the consequences of them not being source stable, whether you're a leaf target or a package. It's not a question for evolution how any packages that do decide to do this deal with the fallout (there are various techniques that can be applied such as adding #if to an existing tag – again, not a topic to discuss on this thread).