“GFW exempts a connection if its first payload has more than 50% printable ASCII.”
Shadowsocks, VMess and Obfs4 were built so every byte looks random. Since November 2021 China blocks them in real time on that randomness alone, using crude entropy and ASCII tests. The rules would block about 0.6% of normal traffic, so the GFW only applies them to 26% of connections to popular data center ranges. Looking like nothing is now a fingerprint.