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).
In the mean time, it would be great to continue the discussion about how to smooth _modify's rough edges. The best solution for everybody would be if we could improve it to the point where it can become an officially-supported feature.
Is there any place I can follow along to see if there is movement towards removing the underscore and making this an official and supported API? I can't find much more discussion about _read and _modify. I see them both used in Swift Standard Library source… but I don't see where I can follow along to see if these were going to become "first class" API on properties (along with get and set).