This was started by Jon Sneyers, co-chair of the JPEG XL (ISO/IEC 18181) group. He said: "Specifically for JPEG XL (or codecs in general), a free spec makes it a lot easier for external enthusiasts to make an alternative implementation"
Yeah, this is something that so many of the comments here seem to miss.
Simply having to pay for the standard is an enormous barrier to access for anyone doing anything of a non-professional nature with the information.
It almost doesn't even matter what the price tag is. Just the fact that you can't say "read the spec, here's a link" to someone with casual interest in a forum post is enough.