Turn off two factor authentication

Two-factor authentication is enabled by default on new Apple IDs and cannot be turned off.

Read the entire section for what Apple says about turning this off since in some cases you can still turn it off (an earlier quote was incomplete):

If you have an old account (several years old) and recently activated Two Factor Authentication note this section of an Apple support article:

"Two-factor authentication for Apple ID" - https://support.apple.com/HT204915

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

... If you recently updated your account, you can unenroll within two weeks of enrollment. Just open your enrollment confirmation email and click the link to return to your previous security settings ..."

Two-factor authentication for Apple ID

"If you already use two-factor authentication, you can no longer turn it off."