How do I turn off my two-factor authentication?

From the article below updated on 24 Apr 2020. 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.       

Note as well that several Apple services require 2FA with your AppleID, so if using any of those you must use 2FA

Apple Card

Apple Pay

Messages in iCloud

iCloud keychain

Home Kit