the official language site Index of /swift-book/LanguageGuide point always to the most current version.
You can't figure out whether one thing is available in older versions of Swift when you reading the chapters on the site mentioned.
Take the example of "Concurrency". If you still develop with Xcode 12 under OSX 10.15 Catalina, there's no way to recognize that Tasks etc. are not available in the version of Swift which comes with Xcode 12.4.
Is there any way to point out the differences or generate a kind of marker which version is at least required to use the features which are described in the docs?
The revision history remarks are not enough in my opinion.
Or is there an doc archive which I still haven't found?