Apple removed the option to turn off two-factor authentication on some Apple IDs created in iOS 10.3 or macOS 10.12.4 and later.
A couple of years ago, I forgot the question/passphrase sequence for two-step verification and subsequently got frozen out. I had initially set it as samephrase1..2..3 in an effort to refrain from supplying PII. In order to reset, I managed to opt-in to 2FA and then revert back to initial setup.
I would have continued to think of the above process as the norm, until I read your comment and followed the support link provided by the other commenter, which states that the 2FA process cannot be undone anymore! However, there seems to be a slightly convoluted alternative i.e. unlinking existing AppleID and attach a new one for iCloud only, thus keeping the (old) existing one for the App Store and other services.
https://support.ikeymonitor.com/hc/en-us/articles/1150008243...