I anticipate an objection to my idea would be that library owners want to be able to opt-in to ExtensibleEnums and immediately start reaping some benefit, even when they have existing clients that are not going to opt-in to a new language mode any time soon. Ideally, if the library owner introduces new enums, they should start getting the benefit of extensibility for those enums regardless of the language mode of their clients. But I also think that there should be a migration path where those clients to eventually start treating the existing enums as extensible too, so I don't think re-use the existing @frozen attribute to maintain source compatibility is great, as it locks in non-extensibility.
What if there were a @preExtensibleEnums attribute that can be applied to existing enums as part of updating a package to use the ExtensibleEnums feature? This attribute would essentially indicate that the enum should continue to be treated as non-extensible, but only for clients that haven't yet opted-in themselves. That way package owners don't have to commit to non-extensibility for existing enums indefinitely.