Getting rid of the GIL isn’t the hard part.

It’s getting rid of the GIL and having existing Python code continue to work, with C extensions and all.

Where is this documented

There's some info on the proposal here: https://news.ycombinator.com/item?id=28896367

I think the root comment is asking whether this new Python-on-Rust interpreter for rid of the GIL. Your link is about the existing CPython interpreter.

Why document something they have not implemented?

Because rightly or wrongly, tons of existing Python code implicitly requires on the GIL to function correctly. Especially when you have C extensions that may not be thread-safe.