Turn off two authentication

From the article below updated on 8 Nov 2019. If it has been on longer than 2 weeks, you can't turn it off. 

Can I turn off two-factor authentication after I’ve turned it on?

If you already use two-factor authentication, you can no longer turn it off. Certain features in the latest versions of iOS and macOS require this extra level of security, which is designed to protect your information. If you recently updated your account, however, you can unenroll for a period of two weeks. Just open your enrollment confirmation email and click the link to return to your previous security settings. Keep in mind, this makes your account less secure and means that you can't use features that require a higher level of security.

Apple ID -Two-factor authentication

After it has been enabled for 2 weeks, you can not disable it. If it has not been two weeks since you set it up, as already mentioned, locate the email you received when enabling it, and use the link there to disable it.

You do indeed have a choice to simply not use Apple services that require 2FA (Apple pay, messages in iCloud, iCloud Keychain). But if you do use those, Apple has the right to enforce their rules for securing access to your data you’ve chosen to place in their systems. Since the are the stewards of that information, and can be held legally accountable if that data is breached, they have the right (as does any online service provider) to enforce security measures they feel minimally necessary to secure your data in their systems.

If that is unacceptable, then you always have the choice to not use your AppleID with Apple services that require 2FA, or don’t use Apple online services at all.