While Google can quickly implement new browser features, other companies such as Microsoft are much slower. AVIF was created as part of developing AV1, which was developed by the Alliance for Open Media. Since all the major browser manufacturers are part of this alliance and everyone is interested in displacing a new and modern format, you expect Microsoft Edge to support AVIF files soon. At the time of writing, Edge does not yet support AVIF images; it refuses to render or otherwise open AVIF images.
If you want to enable experimental AV1 support, you can do so by downloading a plugin. At least YouTube videos will then be streamed in the new format.
Your Edge version
Suppose you are looking for a way to test the functionality and capabilities of AV1 on Microsoft Edge. In that case, you should download the latest version of the browser. The new Microsoft Edge is based on Chromium and was first released in early 2020. It is compatible with all versions of Windows and macOS for whatever reason. Microsoft claims it is the browser you will ever need, but well, who wouldn't say that about their browser. You can quickly determine if you own a Chromium version of Edge by looking at the browser icon. The new version of Edge has a more minimalist and gradient style, while the old one looks like a Van Gogh.
The AV1 Extension add-on is available from the Microsoft Store. It provides support for AV1 videos on Windows 10 devices and Edge. It also allows viewing AVIF images, including thumbnails, and editing in Paint.
In addition, this extension enables video apps installed on Windows 10 to play videos encoded using the AV1 video encoding standard developed by the Alliance for Open Media. As already mentioned, there is no support for AVIF images yet, although avif.io converts images seamlessly in Edge. However, since Microsoft Edge uses the same Chromium engine, support for Edge should be introduced soon.