Do you mean that adding a new keyword to the language, guarded by a
language extension, really deserves this strong wording -- "effectively incompatible"?
Do you mean that I am required to look for this in every program so I know what *actual* language this program is written in? Do you mean that people who have years oof experience with a lnaguage should fully expect to have to look for new magic that means this program is not actually written in the language they know?
Do you mean that stability is completely meaningless when you can pretend that adding a pragma justifies changing anything you feel like?
Or do you mean that stability simply is not a thing at all?