You can just about throw a stone at any terminal in the northeast and hit the next one up the road. Slip seating probably isn't as necessary when terminals are so close to each other.
That being said, I know for a fact that the only way a FedEx Freight driver gets no slip seat is if they've been around 25 years and get themselves a reward truck. And that policy is waived if you happen to be at a terminal that is busy enough to suffer chronic equipment shortages.
Afaik, ABF slip seats because the hostlers do 100% of the yard work as per the CBA. YRCF slip seats for similar reasons. Don't know about UPS, their drivers are dealing with a lot of the linehaul being contracted out. OD slip seats between city and linehaul, as does XPO.
Holland has assigned tractors, not that it prevents anyone from driving anyone else's tractor if city is short on equipment or another tractor is in the shop.
When you don't own the truck, you have very little control over who drives it. Some guys use tricks, hide keys or park trucks in out of the way places. But the only time a company 100% doesn't slip seat is team units.