Effect on local files if I kill iCloud backup / sync / storage

I have seen conflicting responses to my question.

I have two iPhones, an iPad, and a Mac. Apple seems to feel that I deserve only 50 MB (or whatever it is) of iCloud storage, irrespective of the number of devices I have.

Some things are 'synced' among some of the devices. The things I'd like to be shared -- contacts, for instance -- are not. Things that I don't want to be shared are.

I have read that if I shut down my iCloud storage, Apple will delete 'synced' files on my various devices. That sound illegal, and certainly unethical; it sounds like a way to bully people into paying for storage they don't want.

Can I or can I not shut down 'my' iCloud storage -- or at least stop paying for incremental storage -- without losing files on any of my devices