Services talking to each other, and storage persistence, are only tangentially related to internally defined enums. At some point you need to marshal data in and out of a serialization boundary, and it is at that point that you must handle cases you didn’t anticipate. But it’s just serialized data; it may be intended to represent the same value your enum describes, but it’s up to the deserialization code to do the right thing if it encounters a value it doesn’t recognize.

What I mean is, code that deals with serialization cannot by definition avoid the problem of “what if the data is invalid”. It’s not just enums but every aspect of your type system that must deal with this problem (typically by just throwing an error if the data is invalid, etc.)