To expand on it, there's no assurances of that flag working, and crates relying on it to work on stable are breaking all compatibility guarantees the language restricts itself to. Updating stable compilers can break your crate, and nightly features in stable and beta do not receive backports and them being broken won't stop a stable release, so it's pretty much like running on nightly unless you know what you're doing and take on the responsibility of maintaining the stability assurances yourself.

That being said, it can be useful in closed source repos for specific circumstances.

It would be nice if we changed what the flag is named every stable release as a way to "discourage" its use. I wonder if I should write an RFC.