[no_wizard]

Proper deprecation procedures. you can document how you uniformly deprecate and remove APIs. This is a strength of using something like OpenAPI for documentation, or GraphQL, for instance. It is then the responsibility of a consumer to deal with these deprecation(s). On the most basic level you could also do versioning, though its not my recommendation

Document and set expectations accordingly. I've done this move before breaking apart a monolith into separate micro services and this is key. Spending more time on good documentation is generally a good idea regardless.

I'm assuming we're not talking about public facing APIs. That's a situation where versioning might make a lot more sense.