As @Ben_Cohen said,
... 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.
I would assume that the assumption that _modify is a source-stable feature constitutes programmer error, regardless of how "canon" one's package is to the Swift ecosystem. I like swift-nio's shim: a function meant to emulate the behavior of the _modify accessor in a way that meets the library's needs without resorting to unstable underscored keywords.
If a library author must have the behavior of _modify as it was in a particular version of Swift, that's what #if is for, and should suffice as long as that version of Swift is available to compile against. Once _modify is removed in some newer version of Swift, library authors should transition to the new syntax that then-modern Swift will provide.
I may be missing something, but I don't see this issue as any different from the many syntax transitions that Swift has undergone at every major milestone in its development.