I think this is an interesting question: is GHC2021 something that will, for example, just start appearing in a default cabal/stack file? Or, is it the long-awaited -XKitchenSink that I can just enable (explicitly) to imply this common/convenient set of extensions?
I believe that the agreement, based on the proposal that's been accepted and the discussions around it, is that this is very much not a `-XKitchenSink`-type extension. This is a set of reasonable defaults. GHC2021 is the way GHC advertises it should be used (in 2021). Something anybody can turn on without a second thought. I don't know if it ought to appear in default cabal files, though it would definitely be reasonable according the the extension's goal.