> Dependencies on other unstable features

AFAIK that’s not a blocker for Rust - the std library is allowed to use unstable at all times.

I think they meant on unstable features which might yet change their semantics. A stable API relying on unstable implementation is common in Rust (? operator, for example), but that is entirely dependent on having a good idea of what the eventual stable version is going to look like, in such a way that the already stable feature won't break in any way.