I find this an annoying meta-argument because it's so common nowadays, especially on the right. It's really convenient that you don't have to make your actual argument, but can elevate it to the status of desirable secret knowledge by pretending that it would be censored.
I think part of the source of this suspicion is that the abolition of slavery was a result of the Enlightenment and the rise of natural law as a source of ethics, and virtually every aspect of the West has abandoned natural law as a source of ethics or principle, leaving modern ethics in a drunkard's walk around the positions last reasoned to before that jettisoning.
It's common on the right because it accurately describes the state of political discourse today. One small example is immigration policy. I've found it impossible to have a frank and honest discussion, based on facts and logic and opinions, with anyone on the political left. Typically, within two minutes, they'll get angry and call me a racist.