I think installing older software may not slow down a computer as much. Some old software were compatible with older computers with less RAM and slower CPUs.
Some older and slower computers come with old operating systems like Windows XP which can run older games and software which may not be compatible with newer versions of Windows like Windows 10.
Older hardware like old mouses and keyboards, printers, etc sometimes don't work with new computers because it does not have a PS/2 port or parallel Printer port which are mostly found on older computers.