Open Radar

 
So it turns out that, despite the wording on <https://support.apple.com/en-ca/HT204230>, two-factor authentication is now required in order to enable iCloud keychain on iOS. 

My account at the time did not have 2FA enabled. As I detailed earlier in FB8965544, each attempt to enable Keychain would result in the switch immediately turning itself off after each attempt to turn it on with no error message. 

So, one by one I logged out my other devices from the above Apple account (and manually removed each device from the list in Settings > iCloud) until only my iPhone was left. Only then, after once more attempting to enable iCloud Keychain, did it actually throw the message that 2FA is required. 

Reproduction:

1. With a non-2FA Apple account, sign into multiple iOS devices.
2. Attempt to enable iCloud Keychain and witness it turning itself off without an error message.
3. Sign out of all but one device, do the same as step 2, and see that a message appears.

Ideally, the message should appear no matter how many devices are signed into the account.