>>146133JPEG-XL will only work on the preview version of safari, the nightly version of Firefox (requires flag toggle) and desktop Opera (requires flag toggle). Suffice to say, the format is unviewable for over 99% of users and it's unlikely to reach even 50% any time soon.
In addition to that, most OSs won't even preview it or be able to open them by default.
I love JPEG-XL, and I want it to destroy WebP and AVIF before either can pretend they're the image format of the future. But I can't think of a pragmatic reason to want them added to an imageboard as an image format. Even formats as old as WebP are controversial, in terms of convenience and usability when people download them.