When a user with some right to see deleted content (deletedtext, deletedhistory etc) uses the revision API, deleted entries are returned and caching is set to private. This is usually not needed and should be possible to disable so that API results can be cached.
This is a generic problem that probably affects a few other APIs as well, but we have to start somewhere, and for the revision API there is a concrete use case where this gets in the way (T97096).