[nine_k]

No, once-valid input can be rejected after a period of depreciation.

What actually makes sense is versioning your interfaces (and actually anything you serialize at all), with the version designator being easily accessible without parsing the entire message. (An easy way to have that is to version the endpoint URLs: /api/v1, /api/v2, etc).

For some time, you support two (or more) versions. Eventually you drop the old version if it's problematic. You never have to guess, and can always reject unknown fields.

[ljm]

This would also be easy to do if an API is designed around versioning from the beginning, because often it isn’t and it requires a lot of boilerplate and duplication, and it just results in everything being slapped into v1.

Especially the case in frameworks that prescribe a format for routing.