Right, so for an existing package with public enums, there is no migration path to adopting such a feature aligning the two language dialects that doesn't end up with an API-breaking change at some point.
And—since the point is to align language dialects and not make more of them—we'd want this feature to become non-optional as part of some future language mode.
So, if you're a non-resilient package that already exists and has a public enum, the moment we state this as a plan of record, either you are committing to be frozen in time (ha) or you're fated for an API-breaking change.