iOS, sure. After a certain point, you need to be fine with simple functionality (but, I’ll add, more than adequate for most users, which means someone else could find use and value for what you treat as junk).
But macOS? What software? Everything I’m thinking of is graphics adjacent and significantly benefits from faster hardware. For almost everything else, a browser suffices.
No, what Apple made into junk by remotely flipping a switch. On an older iOS, you cannot log into your Apple ID any more, which then means you cannot update the OS any more. So you cannot upgrade, but you also cannot use the old OS for anything that requires you to be logged into your Apple account (which is practically everything). But you still get nagged at every turn that you need to log in and upgrade!