On 8 October they release Microsoft Edge 155, based on Chromium v155 which came with JPEG XL support. Microsoft even mentioned JPEG XL support in the release notes for Edge 155.

It didn't appear to work, however. It appeared the support wasn't (entirely) there because on tests such as this one it showed that Edge 155 didn't support it. Brave browser, which also follows Chromium closely, updated to Chromium v155 a day later and that introduced JPEG XL support, without even any mention in their release notes.

The following day MS removed "JPEG XL decoding support" from the Edge 155 release notes.

Does anyone with a bit more technical insight into Edge and Windows have any idea what is happening?

MS clearly wanted to release Edge with JPEG XL support or they wouldn't have added it to the release notes. There is an official JPEG XL Image Extension to support JPEG XL in Windows 11 in the Microsoft Store and, allegedly, after installing that JPEG XL also works in Microsoft Photos.

Is the code there in the browser but just disabled behind a flag? Are they waiting on Webview2 to also support JPEG XL decoding across all Windows apps? The release notes for Webview2 Runtime 155.0.4283.45 do not mention JPEG XL support, even though presumably it is based on Chromium 155. Does Webview2 v155 already support JXL?

The next Edge update is scheduled for around 22 October. Any chance it might be enabled then?