Censorship, content-filtering, etc. occurs all the time. It occurs within organizations as a matter of IT policy, but also as a way for management to control and monitor employee performance while on the job. Corporations have a right to monitor you and enforce boundaries on your behavior and the use of their equipment on the job, and they often do.
Above the subnet, organization or corporation level, the censorship occurs in the form of investigations, lawsuits and legal challenges, formal DMCA takedowns, as well as active content and traffic restrictions and mitigation of service, both formal and unacknowledged, by certain providers (google EFF Comcast).
On a more fundamental level, the move to eliminate Net Neutrality is de facto censorship in that traffic queuing and prioritization impacts system performance negatively for certain categories of content providers, coercing and channeling traffic towards capitalized providers. While not a formal restriction on content, it would serve to impact patterns of information transfer (read: consumption) as effectively as censorship.
