No I told the story of developing applications. How far back is software made for OSX compatible? Does Safari work on System 7? Eventually everybody has to draw that line in the sand, downward compatibility is a lot of additional labor for steadily decreasing return, this is a platform independent problem. The only real advatage open source has on this is that while the guys "in charge" of a product can set a cut off point if there are enough people in the extended community that want the new rev to support some antique OS then they can do it themselves. But even that isn't a real solution, that's a community based work around that assumes there are going to be enough skilled people that care to extend the product's functionality beyond the line originally intended.
An OS is just a big application.
How far back is software made for OSX compatible?
Most big software requires at least 10.2, which is where OS X somewhat stabilized. OS X itself was compatible with OS 9 only through a sort-of virtual machine, and is now compatible only with an API that is pretty close to the OS 9 API (OS 9 programs must still be partially rewritten to work on OS X).
Does Safari work on System 7?
Only OS X.
Essentially, Apple did what you're talking about. They realized that compatibility would be a nightmare, slowing things down, increasing errors, and resulting in bloat. That's why they started clean with OS X.
Microsoft could have done that with Vista and had a much better operating system in the end. Six years to make Vista. They could have written a new system from scratch in that time that uses the .NET APIs (killing Win32 and MFC). And instead of working on native backwards compatibility have those people work on a seamless VM (they already have Virtual Server) with embedded XP for those who need backwards compatibility.