I thought about this too. My idea was to introduce some sort of version badges, like 'This client supports feature set 2018'. Every 2 to 3 years there should be

I thought about this too. My idea was to introduce some sort of version badges, like 'This client supports feature set 2018'. Every 2 to 3 years there should be new definition of which XEPs a client should support in order to be allowed to be called 'Supports feature set xxxx'. That way you could very easily coordinate a common target for XEPs. Clients which would not support the newest features would still work with the others but users could simply see which clients support the state of the art features. And for developers it would make it easier to identify and skip old XEPs (e.g. how many XEPs are there to confirm a message has been received? Which is the recommended one for new implementations?).