Has apple ever blocked or threatened anyone for developing a rival OS? For an OS that could run on iPhones to be developed, it seems pretty clear that Apple would absolutely have to "support those means" in terms of publishing low level hardware and security chip specs. And distributing secret keys or offering an agnostic signing service.

But what I think you mean to propose is a restriction at the software level, not hardware level. That anyone who sells an operating system must allow any app to be installed within that OS. I think we leave it as an exercise for the reader to define "operating system" when it comes to our increasingly "smart" homes and cars.