When you export an image from the editor, select the file type 'HEIF / AVIF' or use the extension '.avif.' A prompt will appear in which you will be able to choose between several possible options on how the AVIF file should be saved. Since then, thanks to the encoder libheif, it is capable of supporting the AVIF image format natively, which means you can import and export AV1-based AVIF images as you would any other supported format.
This release is titled a bug fix, however it had severe effects on GIMP's functionality. As GIMP 3.0 development has been going on, on the 7th of October 2020, GIMP 2.10.22 was released.
