patterns for expressing usage of deprecated features in a REST response

Does anyone have a pattern/convention they use for advising API consumers in a response that something they did/used is deprecated?

I know you can mark things as deprecated in an OpenAPI spec, but not everything (e.g. if you have a `oneOf` for a field you can't note which usage is deprecated) and I'd like to let consumers know without waiting for them to get/use an updated version of the spec.

I noticed there is a standard HTTP header "Warning" but that ironically this is itself deprecated.