I had epiphany on how the broadcasting a stream without cheating can be done, and it's a bit more simple and obvious. Have the streamers run a second copy of the game that is in "spy mode" that hides the UI elements. So in "spy mode" it just clones the gamestate but removes the UI buttons and task list. This can also then be used with mobile users so they can play the game on a mobile device but someone else can stream them.
True, but the goal is remove the unintended cheating from shoulder-surf (or stream snipe) by watching everyone streaming the same game. As soon as someone dies, it does come back to "did you see who?" and the victim would already know who it was if they were watching carefully.
Especially from spectators to the stream. For the streamer, they can hide their twitch/youtube chat while they play so they're not tempted, but they can't keep people from watching multiple streams to sus out who the impostors are just by looking at the names on top of the characters or the voting screen. I was watching one set of twitch streams the other day, and it's like within a minute people in the other chat windows are already going "it's (other streamer)". So the idea here is to make sniping the stream unreliable.
Another way to accomplish the same thing, albeit requires a lot of effort on the part of the developer, would be to generate a "camera" and "microphone" output from the game and the streamer can then map their microphone through the game through discord or OBS or however way they want to stream it, and just send an alternate image/audio during the parts that would spoil the game. Like a greyscale mosaic over dead bodies. Creating a second copy of the game to run that just relays the info without the spoiler data is easier for streamers to understand IMO.
While I think the audio unintended cheating/self-spoiling is a problem, at least when you play with people you know, on twitch, that can be rectified by having someone controlling the mute buttons that's watching the stream but not participating.
It would take too much work and a "second copy" if the game running at the same time as the first would be A LOT harder than you think.
I get your reasoning, but there are probably other solutions to this problem. For example, if streamer mode is on names and colours are randomised each round. Or the streamers could just ban all colours and names of the streamers they are playing with from the text chat (stop people from saying anything about who it is)