I've been a lot of people bring up jpegxl over the past few weeks/months. When I looked into it, all browsers that had support hid it behind a feature flag. User's have to go way out of their way to enable it as it's not ready for prime time yet. I think that falls under the "still in beta" category.

Safari is not giving anyone a worse experience by not supporting jpegxl yet, as no one should be using it outside of a tech demo, since I'm sure 99% of users will not have support enabled.