Comment by denormalfloat

That's not how HTTP works. HTTP is a transfer protocol, and is not so opinionated on the semantic meaning of the Data it moves. HTTP, HTTP/2, and HTTP/3 were designed to be backwards compatible. It would break that compatibility if there were a semantic change in the absence of the UA string. Specifically, H/3 requests couldn't be downgraded to HTTP/1.1 requests by proxies. There are a lot of proxies.