This format appears to be very new, and it doesn't look like there's wide support for it yet. It would be helpful to know the following:

1. Minimum ImageMagick version required
2. Does a Debian package exist for the reference library?
3. Are there alternative libraries?
4. What browsers, if any, support or plan to support this format?
5. Does common image generation and editing software support or plan to support this format?

1. Per https://github.com/ImageMagick/ImageMagick/commits/main/coders/jxl.c , first commit that introduces JPEG XL is on 2019-10-05 (release 7.0.8-68). The reference library is used from 2020-12-27 (release 7.0.10-54).
2. No Debian package in the repository yet, although sources are available on https://gitlab.com/wg1/jpeg-xl/-/tree/master/debian
3. ?
4. Google Chrome ships with experimental support at least on Windows/Linux/Mac/Android. Firefox is working on it.
5. GIMP plugin is already available with the reference library: https://gitlab.com/wg1/jpeg-xl/-/tree/master/plugins

I have very much been looking forward to JPEG XL support on Wikimedia Commons.
Currently the only way to publish wide gamut pictures (>8bits) is PNG (lossless completely inappropriate for natural content) or TIFF (also lossless and cannot be displayed in-browser), so in most cases I publish as sRGB JPEG and lose all the extra color information that current cameras capture.

all the extra color information that current cameras capture.

What about AVIF? What about ICC profiles?

In T270855#7055698, @VZpolodov wrote:

all the extra color information that current cameras capture.

What about AVIF? What about ICC profiles?

Whatever free modern format gets implemented is fine with me, so long as we finally get one. ICC profiles are everywhere but it doesn't help when JPEG has only 8-bit color depth; using a wider ICC profile with small bit depth results in more color banding.

Just FYI: 14 days ago ImageMagick already fixed JXL integration since in 0.5 version JXL now requires setting color space info. JXL can preserve JPEGs losslessly! https://github.com/ImageMagick/ImageMagick/pull/4064

As for AVIF, the biggest problem is that it does not yet have center chroma siting for 4:2:0 used in JPEG. At all. https://github.com/AOMediaCodec/av1-avif/issues/88

Also, please remember that AVIF needs nclx atom to be set correctly: for sRGB to transfer sRGB, primaries BT.709 and matrix that is in JPEG is full range BT.601. Very important.

Appreciate the heads-up, but I am pretty sure the very relevant follow-up would be: which version is Debian stale on? The hesitancy of the team to even backport testing/unstable source packages has been a persistent roadblock to us getting good things.

Debian 12 has libjxl 0.7 integrated into its repositories.

And Apple has recently announced support for the format in Safari 17, since the last time this issue has been commented on: Google has dropped support for jpeg xl in Chrome, Firefox's implementation is still not ready.

In T270855#8921612, @nathanielcwm wrote:

Debian 12 has libjxl 0.7 integrated into its repositories.

And Apple has recently announced support for the format in Safari 17, since the last time this issue has been commented on: Google has dropped support for jpeg xl in Chrome, Firefox's implementation is still not ready.

WMFs thumbnailing system, Thumbor, is on Version 10, Buster. There is an team working on getting to Version 11, Bullseye, but not version 12. For version 12 Thumbor needs an permanent maintainer, not just a working group like currently. This is blocked on T294484.

And as described in the https://www.mediawiki.org/wiki/Manual:Adding_support_for_new_filetypes you also need file format detection and a media handler. Thumbnailing via thumbor/imagemagick alone is not enough.