We'll see if this long rambling post I was typing as we were moving works or not.
I've been through at the south entrance at least once in the last few years (last year) where I came in at West Yellowstone then circled around and came out through the south and through Teton Park to Jackson. As I remember it I think you're right. I think they charged me an amount without any questions when I entered and I don't remember having to stop at the exit (south) from Yellowstone or the entrance to Teton. In contrast I had another trip 6 or 7 years ago where I came in at West Yellowstone and specifically remember them asking me if I was going to both parks and being told that Yellowstone alone was X and if I was going to both it was Y, but that was cheaper than the individual entrances to both. That trip when I left via the south entrance I had to show them my receipt to get into Teton.
But I'm not sure any of this matters. The infrastructure for the entrances and exits still exist, even if they aren't manned by money collectors. If they have cameras there I'd expect they would still be there and while possibly more prone to issues (not getting as much attention or maintenance, maybe) still getting the video captures they were before, I'd think.
MOO
ETA: Wanted to add that your first line was premonition.