> For the purposes of this discussion the question is not whether or not a crate exposes optional features that require a nightly compiler, but whether or not a crate makes use of the nightly compiler mandatory

In my opinion what matters is the functionality. If it's provided by a nightly-only crate or as a nightly-only feature of an otherwise non-nightly-only crate it doesn't really matter.

But I agree that this is become more and more rare.