Which iOS features require two factor authentication?

Today my iPad automatically updated its OS to 14.2 and it shows a prompt from Apple ID Security to enable two-factor authentication.

The message mentions that "Some iOS features require two-factor authentication", but I could not find which features exactly require it. The Apple's support page Two-factor authentication for Apple ID tells me what this two-factor authentication is and how it is important, but it doesn't say which features of the OS require it. There is only the same vague phrase without any details:

Certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information.

...

You need two-factor authentication to use certain features that require improved security.

I'd like to know exactly what features of iOS 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&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.