Edit: oops, I was wrong. There is an RFC and it's linked from http://www.451unavailable.org/what-is-error-451/
Nginx is totally relevant as many ISPs including our use GNU/Linux boxes running Nginx as a highly performant transparent proxy (there are TPROXY patches for Nginx) to dive into HTTP traffic and do URL filtering (obviously, after initial crude IP-based filtering). Costs less than those fancy Cisco solutions, and it's not like we're willing to spend additional money on something that downgrades the service.
Also, there are cases where actual sites are legally forced to remove resources. Well, not really forced, but it's just a sort request too many sites can't really decline. You either comply and remove a single page (blocking for Russian visitors only seems sufficient), or get blocked on ISP level and since many ISPs (including several giant ones) just blacklist a whole IP address, that means your site becomes completely unavailable.
I hate the censorship but I like the fact you're not gagged and can talk about the fact traffic is being mirrored.
The sad thing is, practically nobody cared about that, for years. The dissatisfaction became visible only when government granted themselves an ability not only sniff on others communications (which is obviously invisible to end user) but also actively censor them.
Non-authorative reference: http://en.wikipedia.org/wiki/SORM#SORM-2
From what I've heard, SORM-2 hardware is a secured 1U *nix-based server (peer was not sure whenever it was BSD or GNU/Linux variant), running some kind of sniffer (probably pcap-based) software with some FSB's in-house tools. They are supposed to be dormant for the most of time, but nobody except FSB knows what they're actually doing (and they don't have to report when they're doing a lawful intercetion).
Think about that! (and submit a patch to support 451 in your favourite web server / framework).