I’m only aware of 3 situations where dealerships service classics today along with 2 others that used to but don’t anymore:

1. The Corvette. It’s GM’s halo car so it gets a certain level of white glove treatment that others do not. It’s also still possible to get any part needed for a Corvette directly from GM.
2. Porsche. Any part can be ordered directly from Porsche for any vehicle they’ve ever sold and they have infotainment upgrades (among other upgrades) available for pretty old Porsche vehicles. They even give a pretty steep discount on regular maintenance to keep owners coming back.
3. Jeep dealerships. I’m talking the ones that have been Jeep dealerships forever, like pre-AMC ownership. Many of those dealerships happily service classic Jeeps and even buy and sell them.
4. Mercedes-Benz used to by on the same level as Porsche, and there are plenty of owners of old Mercedes-Benz vehicles who still take them to the dealership, but they don’t encourage it like they used to. Part of it is that their build quality took such a nose dive in the late 90s/early 00s that it became a major headache for a lot of service departments.
5. Volvo used to encourage owners to dealership maintain their vehicles in perpetuity. Once Volvo decided to take on the full prestige luxury brands (like Mercedes-Benz, Lexus, BMW, etc.) they immediately stopped servicing vehicles over a certain age. In their view having a bare bones 240 or 850 right next to an XC90 Recharge didn’t send the right message to potential buyers.