This is a very welcome addition.
JPEG XL (uses a .jxl extension) is the future, this is a fact, at least for photos, there is no way around it long term, both for private and professional users.
I already converted all my jpegs to jxl files, just to reduce file sizes. If I don't like it, I can always convert is back and get exactly the same jpeg as before.
I shoot all my new photos in HEIF, because a JPG converted to jxl is still larger in size than directly capturing in HEIF (For uncompressed conversion, if you compress it you can easily get much smaller and not notice it). I just chose to go with the smaller file size here, the arguments the jpeg people make are all valid though.
Now the only thing left is for Apple to allow us to use it as the main format, without any drawbacks.
If the camera app would capture directly to jxl, file sizes should be much smaller than both JPG and HEIF.
The only problem there might be is live photos... but they can stay on HEIF/HEVC/HEIC.
As for AVIF, the only time it outshines JPEG XL is for very low quality photos and for animation (GIF), and the web has many of those. I don't know how low quality it has to be exactly for AVIF to be better though. I just hope that Google jumps off their high horse soon and implements JPEG XL into Chrome, some people want to serve high quality images on the web after all. Then everyone can choose what format suits their application best.
Meanwhile for the average user AVIF will never be of importance, it will stay a format for the web, and the web only.