There are a lot of true things (especially government programs in the wake of Snowden and ubiquitous smartphones/encryption) that the discussion of which is shut down due to being literally conspiracy theories (ie proposals about the true functioning of the state to enable surveillance and deny people their rights). People don’t like addressing the fact that there are parts of the state that are actively malicious and hostile to the population they are ostensibly designed to serve.

You also can’t generally criticize the police or military or social order beyond the standard “everybody knows” talking points.

Honest discussions of how society should deal with and treat people who are afflicted with pedophilia is another. There are entire swaths of viewpoints which are basically not allowed to be expressed, including most/all of the compassionate ones.

Criminal justice reform and the prison-industrial complex.

Atheism.

Suicide, and people’s rights thereto.

Drug usage, and people’s rights thereto.

The occasional need and right of a populace to employ violent revolution to secure human rights.

The illegitimacy of state violence.

Artistic nudity (I’m a portrait photographer).

Non-artistic nudity (porn is great).

Artistic non-mainstream porn (weird BDSM stuff is great).

Individual responsibility of people doing immoral jobs. People get absolutely outraged at true reporting of facts (“John Doe, whilst at $EMPLOYER, implemented unethical feature $X”).

Criticism of group identity, or discussion of the legitimacy of identity politics in general.

Jokes or memes about any of the previous, or sexual violence, or tragic deaths.

Increasingly, anti-war viewpoints. The global west is heavily invested in any enemies of Russia or China being unswervingly supported. Being anti-war is more and more a fringe position that is subject to censorship.

Criticizing the major publishing platforms, on those platforms themselves. You won’t be able to run an Amazon workers union site on AWS, for example. Nor can you discuss CCP censorship on TikTok.

Lots of things are off limits in most parts of the internet. People who think the only people complaining about the restrictions on speech online are nazis or pedos are... fairly unimaginative.

Many different areas and topics. The most concerning to me is bans and shutdowns of public forums where there is discussion of how authorities are disregarding human rights as defined by the UN in the UDHR.

I’m not sure how you expect to answer that without access to the inner workings of content moderation decisions on the major social media platforms. Sunshine laws would be a good starting point for productive discussions.

In an absolute sense, I can talk about anything as long as I am willing to muck through the mud of low quality noise.

In terms of having actual discussion, sneak laid out a good range of topics that quickly get derailed in flame wars (IME). I'll personally add homelessness and mental health as a few more very touchy topics.

I know high quality, nuanced discussion isn't a 1st amendment right, but that's what the other half of the conversation wants while most argue with the strawman half that wants to be racist/sexist online.