I wasn't clear, I mean without recompiling. Your suggestion is a compile time check.
You said "I would like peasy to continue to work on 1.28", which it will, without change. The compile-time check would only be needed to work correctly on 1.28 and 1.29 if this change is merged.
Why didn't we merge geany_api_version() that was suggested a while ago?
I don't know what that is, but assuming it's the run-time API version, you would still need to change your proxy plugin to use it.