I think they do. There are machines for which there isn’t any version supported on that machine, though.

That’s not different from the situation with Windows. An Intel 386DX CPU with 4 MB of system RAM and 50–55 MB of hard disk space won’t see Windows security updates, either, anymore.

What is different is that Apple is less clear in support periods and often more aggressively leave older hardware behind.

I don’t think you can require them to support things forever, but requiring everyone to be clear about it at time of sale, or requiring X years of support would be good ideas, IMO.