I was actually working on that recently when I uncovered this hole, which I also hope can be patched in Swift 6.
That said, I think non-frozen enums are an additive change. I highly doubt we are going to change the default behaviour of enums, so you're going to have to opt-in to extensibility via some kind of @nonfrozen attribute (although now we have a notion of cross-module-but-same-package in the compiler, there would likely be less breakage than previously expected).