Regarding explicit guarantees, I'd consider a "forward-compatible" way such that when - and if - a breaking change needs to be introduced, it's possible to automatically convert old JSON schemas into the new one. JSON Schema version which would select which processing to choose could be an option, but not sure if there couldn't be better.

As is often the case, good domain knowledge can help to choose which features are "real" and can stand and which are more doubtful and have a higher chance to fall out of favor later.