> My understanding is that AlphaZero only really existed for a year or two;

It still exists, but it's private / internal, and sometimes used for a few different things.

It was used by Kramnik to test the hypothesis whether no-castling chess was viable (basically chess, but disallowing castling). That was a year after DeepMind published the match they ran of AlphaZero versus Stockfish.

It was still in use last year, I remember seeing some Grandmasters with interests in chess studies were invited by DeepMind to judge the beauty of chess problems composed by AlphaZero (or whatever form the thing that used to be AlphaZero is now).

So it's still kicking around in the background.