I think that's true but South Parks humour has also changed over time. That's the nature of satire - it lampoons human folly and vice, including the ideas of offence and moralising which are so often borne out of hippocracy.

You mention taboo topics like 9/11 as if it's a no go area but actually that has been a rich source of comedy and satire due to the level of hippocracy displayed around it. The hippocracy of Uber patriotism, religious nationalism, racism (you mention people having to be careful about the target and culture of jokes, but many groups found the exact opposite after 9/11 - certain ethnic and religious groups were all tarred with the same brush, particularly in the US) and more. Even the idea of self censorship out of fear of causing offense. Some of this is being replayed right now with a contemporary conflict.

South Park is in a similar tradition to other satire such as Private Eye in the UK, or The Onion, or various other TV shows. South Park is just a sometimes more extreme version more willing to be deliberately offensive. But satire moves with the times like any other type of humour.