Opt-in should be mandatory for all services and data sharing. I would start my transition to Linux today if this were opt-out, though the way Apple handles this for other services makes me believe opt-in will be temporary.
Currently, when you setup any device as new, even an offline/local user on macOS, the moment you log into iCloud it opts-almost-every-app-and-service-into iCloud, even one's you have never used and always disabled on every device. There's seemingly no way to prevent this behavior on any device, let alone at an account level.
Currently, even though my iPhone and language support offline (on-device) Siri, and I've disabled all analytics sharing options, I must still agree to Apple's data sharing and privacy policy to use Siri. Why would I need to agree to a privacy policy if I only want to use Siri offline, locally on my device, and disable it from accessing Apple's servers or anything external to the content on my phone? Likely because if you enable Siri, it auto-enables (opts in) for every app and service on your device. Again, no way to disable this behavior.
I understand the majority of users do not care about privacy or surveillance capitalism, but for me to trust and use a personal AI assistant baked into my devices OS, I need the ability to make it 100% offline, and fine grained network control for individual apps and processes, including all of the OS's processes. It would not be difficult to add a toggle at login to "enable iCloud/Siri for all apps/services" or "let me choose which apps/services to use with iCloud/Siri, individually". Apple needs stronger and clearer offline controls in all its software, period.