In the US?
Jail no.
Freedom of speech, the first amendment, only means you are protected by law from legal prosecution/persecution for saying disagreeable things.
You can certainly deny the Holocaust, or 9-11 or say the N-word (as a non-black person), but it doesn't mean you can say that stuff without consequences.
But the rules are far more complicated than a foreigner might imagine.
The consequences of anything you say depend on who you are and when you say it.
The rules actually change from day to day. What is acceptable yesterday, may not be acceptable tomorrow.
And what is unacceptable today MAY be acceptable tomorrow. It is really hard to say for sure.
This is why even professional pundits get into trouble ALL the time.
Usually, saying the wrong things often means getting fired from one's job, and becoming unemployable (for an arbitrary amount of time. For example, Bill O'reilly is still unemployable. As in Megyn Kelly. They have to wait until the internet has forgotten them... ).
These rules are very complicated, and basically anything can offend someone.
If you are afraid of running afoul of these rules, it is best to say nothing at all.
Here are a few examples of what can happen...
His first protest ever cost him his career, reputation and sense of self
Megyn Kelly’s Blackface Comment Killed Her NBC Show.
This stuff happens to regular people too, some times over comments made long ago.
20 Tales of Employees Who Were Fired Because of Social Media Posts
These Social Media Fails Got People Fired
This one I believe is an example of the n-word... that was just in the news a few weeks ago
Harvard Rescinds Parkland Student’s Admissions Offer Over Racist Comments in 2017
If you are an independently wealthy recluse, you can say almost anything without any consequences. The government still observes the first amendment. and if you can withstand the societal backlash, you have nearly total free speech freedom. (you still can't shout fire in a theater)