Yeah, the RFC is explaining what they proposed in 2021. In 2022 that work landed in "nightly" Rust, which means you could see it in the documentation (unless you've turned off seeing unstable features entirely) but to actually use it in software you need the nightly compiler mode and a feature flag in your source #![feature(float_next_up_down)].

By 2025 every remaining question about edge cases or real world experience was resolved and in April 2025 the finished feature was stabilized in release 1.86, so it just works in Rust since about a year.

For future reference you can follow separate links from a Rust RFC document to see whether the project took this RFC (anybody can write one, not everything gets accepted) and then also how far along the implementation work is. Can I use this in nightly? Maybe there's an outstanding question I can help answer. Or, maybe it's writing a stabilization report and this is my last chance to say "Hey, I am an expert on this and your API is a bit wrong".