There's some good discussion here: https://old.reddit.com/r/adventofcode/comments/1h9cub8/discu...

But yes, you would have people openly share repositories for automatically ingesting the puzzle text, solving the puzzles, and submitting the results the moment the puzzles opened, leading to inhuman solution times. So, despite the rules stating that you can't do that, the result is that whenever the puzzles were easy enough for an LLM to solve them with high probability – and most of them are – the leaderboards would be overrun with such solutions.

In 2023, the LLMs would still struggle enough that the overall leaderboard (taking all 25 ⋅ 2 puzzles into account) would still be dominated by ordinary people (many of them recording their solutions), in 2024 that was no longer the case. Personally I would go from being able to top 100 regularly, to almost never being able to. I'm going to miss the thrill, and think it's a bit saddening that we can't have nice things, but also ultimately think that getting rid of it is the best option.