I don't know if this is the case, but I think it's entirely possible to know that shadowsocks is being used widely without being able to do anything about it at

I don't know if this is the case, but I think it's entirely possible to know that shadowsocks is being used widely without being able to do anything about it at the network level. I think that's plausibly how shadowsocks is designed to avoid the GFW in the first place. For example, if their capabilities to identify shadowsocks traffic is not particularly specific, filtering would result in undesirable impact on other traffic. They can also have other out-of-band estimates for the extent of shadowsocks use (presence of the software on seized or searched equiment, observed chatter, informants, etc).