Porsche's classic division is really setting the standards for supporting older cars. They realize over time it'll become an increasingly relevant portion of their business and just increase revenue. You can even send a car out to Germany and they'll fully restore it (at quite a price). The big thing though is they simply produce parts for their older cars.
Mercedes-Benz is also pretty up there for supporting their classic cars. Aston Martin also increasingly building out their support for their cars. Ferrari and Lamborghini starting to get more into the game, but for those brands it very much remains going to a big few shows in the EU to round up parts at times. Toyota, Nissan, Ford, Chevrolet also have some levels of support.
I don't think there'll ever be a need to 3d print shock absorbers or bearings, since those tend to be commodity wear parts and there will probably always be compatible modern variants (or if you look at some parts guides, many current cars still use the exact same basic parts from several decades ago).
Engine blocks and major large internals like conrods or more commonly needed transmission internals are the parts I think will be most difficult to keep stocked.