How to mark an endpoint out of service kind of thing?
Could you explain a bit more what you are trying to do?
I can see the need of noting deprecation for parts of an API (which could be a path or an operation, or even parameters), with a "planned end of service" date or similar.
I have some services that are defined but not implemented that and I would like to have a flag that could inform the consumers of my API about it.
Maybe just return 501 http code for that endpoints?
I think a better solution is to not include those endpoints in your OpenAPI spec.
Or you can provide two versions of your OpenAPI spec.
In any case I don't think these should go to core of OpenAPI spec since OpenAPI represent your API and not your workflow.