It has been "allowed" for a long period of time. But (in my albeit limited experience) would only happen in extreme cases. I know if I sent in a camp permission form to district saying I would have both sexes sleeping together, I would have to make one hell of a case for it. And as a group, we would never plan for it.
But, things happen. Like the flood mentioned above, or a broken tent pole, or any other random situation. We can't just kick all kids off site - we would have to wait till parents (or whoever) could come and collect. We could be 4 hours away - and parents are not the best at answering phones, or staying sober. You can't keep the kids up and awake all night - you need them to sleep. So both sexes in a single space is the best decision in that scenario. So the rules allow for this.
The only planned camp I have been aware of where different sexes shared was a sibling group, and there were other factors at play as well.
As far as trans kids - so far, not had any, so not had to manage that minefield. In that scenario I may plan a lot of backwoods/hammock based camps, so no tents to share! But seriously, I would not want to plan for mixed sexes in one tent, but would have one scout in a tent by themselves (older ones frequently do) so would probably do that.