Queuing videos is okay and when streamers have to take a break to fix equipment or use the restroom they keep it up. As long as chat is actively moderated, you can do anything for the most part. I don't know if there's an unwritten rule about having to show yourself on screen at least once though. I imagine a lot less people would show up or stay if they couldn't see hear or see the streamer.
There's things like study with me streamers where they just read a book or study in front of the camera. There's even sleeping streams where some people sleep on camera. Sometimes they are doing a 24/48 hours stream challenge.
Ludwig, a very well respected creator, did a subathon where each new sub extended the time he continuously streamed by a little. He ended up streaming for 30 days.
I think prerecorded is the only real taboo culturally. There's streams where a creator will say, chat send me things to watch, and they just watch with chat for hours and react. There was a time during which streamers were pushing the limits of DMCA striking and some streams watched seasons of master chef and other popular shows on stream.
This article does a poor job exploring the nuance of the situation or even the diversity of content people make.