Which iOS features require two factor authentication?
All of them if you don’t have a grandfathered AppleID. Furthermore, the matrix of services and features change routinely and also have more to do with geographic constraints and whether your account is managed or enrolled in some select Apple programs.

https://support.apple.com/guide/iphone/manage-two-factor-authentication-iphd709a3c46/ios

I’m sure a Wikipedia list of what’s changing might be better for this than a Q&amp;A site like this. The pragmatic answer is you will be prompted for each service you try based on your AppleID creation process and then only when the server side clears your account for enrollment.

https://support.apple.com/en-us/HT204915

I would engage Apple support if you need this answer as they have the details about your Account and also can check with engineering on recent changes that might affect you specifically.