If your iPhone (or iPad) gets lost and stolen, and the thief successfully changes your iCloud password, you’re now locked out of your own account. (Along with ALL devices that use your iCloud account.)
To prevent this, add an additional passcode for accessing your iCloud settings:
Go to Settings
Select Content & Privacy Restrictions
Turn on Content & Privacy Restrictions switch
Select Account Changes
Select Don’t Allow
Go back to the Screen Time page, and tap Use Screen Time Passcode.
(Best to make this different from Screen Lock PIN)
Doing this will grey-out your iCloud settings. To enter iCloud settings in the future, you need to turn off Content & Privacy Restrictions using your Screen Time Passcode.
(Many thanks to Cult of Mac for the tip.)