Fwiw, my vet, most of whose cliental are beloved cats owned by wealthy people, also does work for local shelters. While I’m sure the shelters never opt for chemotherapy for cats dying of cancer, I’m pretty sure that the only difference in how he treats tame shelter cats and pet cats is that he gives the shelters a modest discount. The cats who are treated for ear mites, respiratory bugs, who get spayed and neutered, get exactly the same care as my pets. (He also spays and neuters feral cats for a trap/neuter/release program, run by the same shelters. Those cats have to be handled a little differently, as they can be dangerous.)
I volunteer at a couple of local shelters, and all their veterinary care is purchased from ordinary local vets who mostly treat people’s pets.