this is insane. The auto update turned it on! 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.

That’s correct. You cannot turn off 2 factor authentication, and I can’t think of any reason to. I go for weeks without having to authenticate anything, and in the rare instance when I do it takes a couple of seconds.

And not just with Apple. More and more services on the Internet now require some form of 2 factor authentication. Essentially your account can be hacked on any site or any service that does not use it. My bank requires it. My brokerage accounts require it. The US IRS requires it. Most other US government services require it. Google enterprise apps require it. Microsoft business services require it. My cellular network provider requires it.

Concerned over device loss? A quite reasonable concern, that. Best set up trusted telephone number(s) and/or recovery email address(es) earliest, then.

You cannot turn it off. More and more features require use of 2FA.