It's quite the other way around: threats must be very specific in order to not be protected speech in the United States. Saying 'let's burn this mother down' during a protest is protected speech, for instance. In general, the threat must be specific and imminent, such as 'let's go to that building right there and do $violence right now!'

They have been mentioned several times in this thread, but you should consult Popehat's writings for in depth breakdowns of what is and is not protected speech.

The doctrine is "imminent lawless action".

https://en.wikipedia.org/wiki/Imminent_lawless_action

Pointing at a person during riots and screaming "Kill that muthafucka now" would almost certainly cross the threshold.

Organizing an unofficial Wannsee conference to plan a genocide of $UNDESIRED_GROUP into details would not, even though the entire event would be a hatefest.