I pitched this very recently and even implemented it:
There is lots of useful feedback in that thread which I don't see addressed in the pitch, including on the naming. For instance:
The same criticism also applies to @extensible.
Personally, I continue to prefer @nonfrozen, but I don't really care as long as the functionality exists under some name. If we do actually manage to change the default behaviour for enums in a future language mode (as the LSG says they are open to), the attribute would just be temporary anyway.
We generally ask that people pitching a feature read past discussions on the topic and incorporate them in to their new pitch. That just saves everybody a lot of time, not having to re-hash arguments or miss some important details.