I'd guess not the latter — anecdotally, my intel mac has been increasingly sluggish after each macos upgrade, even after a clean install. I finally decided to revert to the oldest macos version possible (and then update by just one major version since unfortunately some apps I need don't support Catalina anymore) and the difference is night and day — my memory was right and my laptop _was_ faster before. And it's not hardware getting old or battery problems, it's the OS.

So I don't know if older macs are intentionally crippled or they're just ignored during QA, but I don't believe they're actually intentionally supported