HTTPS://URLs. The world is full of these URLs and it has been deemed impractical and downright unreasonable to introduce another URL scheme for the new protocol. Much like HTTP/2 did not need a new scheme, neither will HTTP/3.
HTTP://URLs will be left as-is and as we proceed further into a future with more secure transfers they will probably become less and less frequently used. Requests to such URLs will simply not be upgraded to use HTTP/3. In reality they rarely upgrade to HTTP/2 either, but for other reasons.