Edit: it's common for OSX to drop support for old hardware. It's also common for software to improve overtime with upgrades and bugfixes. Developers don't want to support old software indefinitely, unless you can do so with newer tools on newer hardware. Simply researching the limits of legacy systems is a massive waste of time, supporting them is a nightmare.

Try to find a modern version of some software on OSX 10.4, for example. Development has likely frozen years ago, meaning bugs won't get fixed, security is out the window, and everything sucks forever.