In some internet discussion, one guy claimed that Prussia of Frederick the Great had greater freedom of speech than modern United States. This is so unbelievable that i asked him for sources, he of course refused to cite any, as expected.
I did some research of my own and found that freedom of speech in 18th century context meant criticism of religion was allowed, not criticism of king, the government, the army and war.
Is this accurate impression? What kind of speech and press would be allowed in 18th century Prussia?
The person you spoke to seems to have a very limited understanding of Early Modern censorship, which was infintely more oppressive than today. As such we will look at two examples.
First, Frederick and Voltaire. While the two became friends, in this article, you can see a very prickly history with Voltaire and Prussia. This is rather common as most countries up until later in the 19th century to have rampant censorship with banning of books or phamplets that they considered untrustworthy, false, or generally something they didn't like for whatever reason. Censorship is less about getting rid of enemies but presenting an image about the Crown and government that's being discussed, but the two go hand in hand.
Conversely, Emperor Joseph II, Emperor of Austria and the Holy Roman Empire was the most progressive of the Enlightened Monarch with a push for education, ending any sort of Serfship, and loosening of censorship. He would mainly loosen censorship in Vienna (but not the rest of Austria) because he says that "the Viennese are happiest when they complain." However, as a result, phamplets and books that spread false rumors about the Emperor spread quickly and the censorship would return.
So, no, Prussia isn't an ideally place of free speech.