I Hope the Dev himself get back to updating the mod again but meanwhile I wanted to share this so you can check the code and what I changed and build a jar file for yourself or straight up download it and enjoy
https://github.com/EGOIST1372/HorseStatsVanilla/releases/tag/1.21.7
it seems it needed a lot of changing numbers of APIs and correcting 2 attribute from GENERIC to non-generic (generic attributes seems to be deprecated)
It also needs:
Latest fabric loader 0.16.14-1.21.7
Latest LibGui v14.0.0-1.21.6
And you good to go

P.S.
Why I didn't made a pull request for it? there are 4 pending pull request from last year