Censorship, beyond what’s required by law, is doomed to fail.
Not exclusively, no. There's nothing in the definitions of the words 'censor' or 'censorship' that imply it is an act exclusive to governments.
Effectively, something can be censorship even if the government is not involved.
When the government is involved, then it's government censorship.
For practical purposes though, the kind of censorship that we're concerned with in this conversation can't be done by anyone other than a government or a lunatic with a gun. Companies just don't have any authority over anyone except themselves. They can't deprive you of your ability to speak, only your ability to use their property to do so.