It is not ok on iOS or macOS or even Android or whatever else. The feature to have some online storage with backup is fine, IMO, but what is not fine is the OS nagging you to use that feature. This thing must be opt-in and only if the user themselves initiate it.

Also it should not be locked to a single online storage provider but use some sort of standardized protocol (or at least some pluggable mechanism) to allow any online storage provider - including using self-hosted options - to work with it.

This is how you make something that works for your users instead of taking advantage of them.