I know the support is still almost inexistant.... Just supported in Windows10 since May, and no browser supports it atm, as far as I know. (There is however a polyfill : https://github.com/Kagami/avif.js)
But the AVIF format was born earlier this year, and support will probably arrive soon everywhere (AOM initiative, from google, mozilla, apple, etc...)
The promise of this format is big, and it should - finally - be a real replacement for our current outdated formats (jpg, ..).

avif spec : https://aomediacodec.github.io/av1-avif/
libavif : https://github.com/AOMediaCodec/libavif