This is what I assumed as well, but online I found people who were able to update a ~2015 MacBook Pro to a version of macOS more recent than the latest supported one, and it still worked.
Both are true. Apple's official qualification, continuous integration, bug fixing, backporting, etc. takes more effort than a hobbyist getting things to (mostly) work. But the main issue for Apple is focus: Apple wants its product development organization to focus on the next new thing, not on the past.
I'm mostly OK with Apple's 7-10 years of support, but I also have older Macs that still work fine and that I wish were safe to connect to the internet while running macOS - without relying on various hacks or workarounds.
