It remains missing from Safari. It used to be offered as an experimental feature, but I don't seem to have it anymore on Safari 16.5.
https://developer.apple.com/documentation/safari-release-not...
You can check with Cloudflare’s test page here:
Alternatively, open up developer tools, go to the network panel, right-click on the headings and enable the protocol field. It will tell you which protocol was used for each request made by your browser. Obviously the server also needs to support HTTP/3 for a request to use HTTP/3.
Safari not supporting isn't a problem unless your business only and specifically targets Safari, and even then it's a relatively painless way to help the few customers who run preview versions get a nicer experience.