Editions are only necessary for otherwise-backwards-incompatible changes, to avoid breaking existing code. Even relatively major language features like the ? operator were shipped without an edition bump. But for example I think the await keyword itself was added as part of an edition, because it was incompatible with existing code that had a method named await.
The messaging around the 2018 edition was a little different here, with some attempts to use the edition release as an opportunity to advertise and summarize all the features that had shipped since 2015. But I think folks decided that was confusing, and with the 2021 edition the messaging was toned down.
Yes, that's a fair point about the actual purpose of editions being to handle forwards-compatibility.
Informally though, I think there are certainly a subset of people that use "Rust 2021" to mean all of the features included up to that point, even if it's not the precise definition.
The messaging around the 2018 edition was a little different here, with some attempts to use the edition release as an opportunity to advertise and summarize all the features that had shipped since 2015. But I think folks decided that was confusing, and with the 2021 edition the messaging was toned down.