I just wanted to ask if it is legal to stream other people's voices in the in-game chat without having explicit permission from them? If people forbid it, the matter is clear, but what about if they don't know that someone is streaming their voice on e.g. Twitch?
Yes, it has to be legal, because as soon as you use the in-game chat you are engaged in public communication. What would be different if it was private chat on Discord or TS and the streamer didn't tell you that he was streaming. Let's say there's a random team in a game and they all use voice chat, one is streaming - is legal. Even if there are only 2 random teammates.