What people seem to be missing here is that enums are just a reflection of functionality. If you make your enum non exhaustive then add a field, and something stops working because it's now missing critical messages, it's still a breaking change!

It's always up to callers to decide to handle everything, or decide that additions are non-fatal and use a wildcard branch to ignore them. The same enum could need to be exhaustive for one use case, or subset matched by another.