I agree that this is usually how it works.
The point is that the authors of swift-system and swift-collections obviously had these expectations when they made use of _modify in their libraries, both of which have released 1.0 (stable) versions. The proverbial horse has bolted.
It seems that that was an error - it happens. In future they (and others) will need to be more careful about publishing releases using unstable language features. But as things stand, there appears to be very little (if any) room for _modify to change, without breaking clients who did the right thing and never used _modify directly in their code.