An interesting difference with the WebP situation is that any app that uses Apple's codec framework on iOS/iPadOS, which is almost all apps that deal with image

An interesting difference with the WebP situation is that any app that uses Apple's codec framework on iOS/iPadOS, which is almost all apps that deal with images, will support JXL automatically. Actually, I just downloaded a JXL via Safari, and after confirming that it was still a JXL, I tried using it in a context where JXL isn't supported, and it automatically turned into a JPEG.