Per this log, looks like the dependency on whatever quilt json library this is is missing on 1.21.5. This may also be an issue on the other forge versions but this is just what someone happened to be using in mcforge's player support channel.
(Hi, I'm one of the moderators over there).
I feel like it would be easy enough to just use forge's config api for the forge impl. unless there's something special about quilt's that you require.