AVIF support was already introduced in darktable 4.2, but as Ansel is based in darktable 4.0 it seems it is not able to export in AVIF yet.
I know that is not extensively used yet, as support for it is quite new in some web browsers, but it is support in many of them and many forums do support it already.
It provides more quality than jpeg for the same compression, I have tested it and you really can see the difference.
It would be great to have it in ansel.
I don't know how programming intensive that could be or if it is just a matter of using a newer library for exporting images, but if the effort is not too much, it would be a great addition.