Forwards compatibility would be old code compiling in the newer "language", and I mean this from the grammar perspective: that the changes to the grammar are su

Forwards compatibility would be old code compiling in the newer "language", and I mean this from the grammar perspective: that the changes to the grammar are such that all things that were previously valid in the language remain as valid, and their meaning does not change. This is not the case. That the compiler has the user opt-in to the never version by setting a flag is irrelevant, for the discussion of versioning. (Now, for an end-user, I think it's great: you do not want to automatically break working code. I'm not saying the flag shouldn't exist, I'm saying the flag exists because the change is breaking.)