I think it very much depends on the circumstances, just like most social rules. If somebody is in an office space, then it can make sense to have everybody on

I think it very much depends on the circumstances, just like most social rules. If somebody is in an office space, then it can make sense to have everybody on camera as the default. On the other hand, if people are working from spaces that were never intended as working space, then it would be rude to assume that they should give a view into their private space. For the first ~12 months of the pandemic, I was working from home, using my desk in the living room because there wasn't a good spot for it anywhere else. Having my camera on would have meant having my living room in full view, often including my partner on the couch in the background. While background detection/subtraction does exist, it isn't perfect, and so that would be an intrusive view into our private area. I've since moved to a different location, with a room to be a dedicated home office. Here, it wouldn't impose on private space to turn on the camera, because that was part of the intent behind that room. However, you as the person on the other end of the call cannot know which situation somebody is in, and so it would be rude to insist on video beyond a polite request.