The specification of AVIF is also publicly available, while you would have to pay a little under $500 to purchase all five of the ISO/IEC 18181 specifications for JPEG-XL. Not a monumental cost, but something prohibitive for a hobbyist:
The C++ committee makes draft versions of the standard available for free[0], I've heard that often the last draft before a new standard version is published is almost identical to the published version. It would be awesome if someone involved with JPEG-XL could arrange for the same. Edit: I found https://arxiv.org/abs/1908.03565 but it hasn't been updated since 2019.
[0] https://eel.is/c++draft/