Is there a typo in the question? By definition censorship is already censorship. It's a tautology.
If you mean when does small scale individual censorship become systematic censorship and oppression?
- in today's internet, with the public square is being privately owned, Twitter Instagram Facebook etc, small scale censorship by the private corporations, can effectively become systematic censorship, because they dominate the digital public square.
This is why myself, and many of my friends, are actively supporting the fediverse so that people's voices can be democratically supported, and not subject to corporate censorship.