I am not sure how would that work? F.ex. how would you introduce tagged unions and make the language 100% backwards-compatible... but not the stdlib?

I admit I have no idea.

The stdlib would remain 100% backwards compatible, but the implication was that he would want to see certain existing features of the stdlib amended with modified versions that leverage the new tagged unions. He imagined that modification would necessitate v2 stdlib packages to maintain sensibility.