Adding a new enumerator to a C enum can change the size of the type,
which would break the ABI.
This is not often a problem in practice, but the rules around when it is a problem and when it isn't are complicated enough that I believe enum should not be used in the stable ABI (possibly with well-reasoned exceptions)

AFAICS, the rules are:
- In C++, an incompatible change to an enum is one that changes the size of the *smallest bit field large enough to hold all enumerators*. Values outside the range cause undefined/unspecified behavior.
- In C, it looks like enums that fit in `char` are safe.

(Also, the compiler-defined size of enums will make it more cumbersome to formally define an ABI for non-C languages.)