If this is to be undocumented I think the language shouldn't have issues that force me to use it.
An unstable implementation detail probably shouldn't be the only way to use/test these — and I would question marking features like Span unavailable in nightly toolchains in the first place, as testing them is kind of the point.
Similarly, this kind of availability does not apply to the embedded standard library, it has next to no dependencies and can't even import Apple's frameworks. Even concurrency is linked statically.
Fundamentally, I don't need this potentially unsafe unstable flag, I need the standard library to use @available correctly.