It would probably depend on the level of impairment and ongoing pain for the cat. There's a cat rescue in my town that currently has two 2-legged cats, and they're trying to rehabilitate both. One cat has a missing front leg and opposite back leg -- it's actually getting around quite well now without additional support. The other is missing both back legs. It's not doing well at the moment, but it only lost them a couple of weeks ago. If the rescue can get enough donations, there are prosthetic options for it.
These two cats are the victims of someone in a nearby town (who is still unknown) who deliberately cut their legs off. While this is probably the most terrible reason for cats to be missing legs, at least it does mean that they had clean cuts and no additional injuries. A cat that lost its legs in an accident may have had shattered bone and internal injuries, so there may have not been any options to restore function for it. It is sad and certainly frustrating, but sometimes it is the only realistic option.