As reported by Jen Simmons, Apple adds support for AVIF image format in new operating systems this Fall.
Support for AVIF is now included in Safari Technology Preview 149.
Preview might be unavailable
Please note that at the moment of publication, build was not available due to issues.
WebKit blog: Release Notes for Safari Technology Preview 149
Developer:Check the support when available.
QA engineer:Check the support when available.
PM/DM:Business as usual.