I remember the 2nd edition coming out in printed form years ago.

Perhaps there was some update in March of 2023, or perhaps this particular set of HTML files was published in March, but overall, the book has been out for a while.

There is a version number at the top: "v8.8.0.8". I believe they keep making updates to the book probably to fix typos/errata. The original link I had bookmarked https://htdp.org/2018-01-06/Book/index.html and this one look mostly the same - content wise.

But it would be nice if there was a CHANGES file or description about what does change between these versions.

The version number matches the latest version of Racket. It's simply the tool used to generate the book site that inserts the version number. It's more relevant in the Racket docs than in the book though.