Wait until you run a PiHole and see all the telemetry and dns requests that are made to *.apple.com even when you are not signed into iCloud.
* Cortana. Nobody should have been forced to use a modern version of Clippy, but at the OS level, and one named after a video game (AI) character. It made the whole OS feel like a joke - to me, at least.
* Tricking/forcing people to sign in with Microsoft/Outlook credentials, so their Windows account would now be tied to MS online credentials.
* Tricking/forcing users to upgrade into Windows 10. This happened to countless people, including to someone in my family.
* Displaying literal ads in the Start Menu
It’s the ads for services or upgrades to service plans or new features of services that I don’t care about that I hate.
IAPs ruined the App Store, services are ruining the OSes.