The way Apple does it, it's cleaner to implement the APIs for this sort of thing in a new operating system version, especially if it requires features not present in the current versions of the operating system.
They could make an exception for Safari by including the required code to support JPEG XL in Safari Technology Preview on Ventura to allow developers to test their sites and web apps until macOS, iOS, etc. ship in the fall.
I suspect we'll learn what the plan is as the Safari/WebKit team make more announcements during WWDC: https://www.webkit.org/blog/14203/web-technology-sessions-at...