There are also sites that previously allowed UDP which started blocking UDP port 443 specifically to block QUIC and force fallback to HTTP/2 or lower. Reasons

There are also sites that previously allowed UDP which started blocking UDP port 443 specifically to block QUIC and force fallback to HTTP/2 or lower. Reasons include some forms of traffic fairness management on mobile networks (to ensure throughout and low latency, it's much easier with TCP), and some corporate network security products to monitor traffic destinations at the domain level.