Incorrect. Library features like the fallible allocation API are available to all editions, for better or worse - it also means you can't make breaking changes to the standard library ever, not even with a new edition.
New language features may require a new edition, though the vast majority of them don't - here's the list of all the changes in Edition 2021 (a new one is released every 3 years): https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html
New language features may require a new edition, though the vast majority of them don't - here's the list of all the changes in Edition 2021 (a new one is released every 3 years): https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html