That’s likely due to iOS/macOS not supporting it in production-default-enabled yet; there’s an experimental opt-in flag at the OS level, but Safari apparently hasn’t (yet) added a dev feature switch for it.

https://developer.apple.com/documentation/security/sec_proto...

Presumably anyone besides Safari can opt-in to that testing today, but I wouldn’t ship it worldwide and expect nice outcomes until (I suspect) after this fall’s 27 releases. Maybe someone could PR the WebKit team to add that feature flag in the meantime?