I work on mobile apps, and I can promise there's no tracking on the apps I've been building if you opt out. The SDKs I've seen seem to do what they promise: no communication to backend when tracking is disabled.
Of course, I can only speak for the apps I've been building and SDKs I've used.
They are not my apps per se, that part is for the company lawyers. I'm just taking care of the technical implementation. It's always some way of opting out/opting in during the onboarding.
You can probably count the ethical, law-abiding adtech firms on one hand.