iCloud: Untrust a saved Web browser
It appears that a cookie (probably X-APPLE-WEBAUTH-HSA-TRUST) is stored when you choose to trust the browser. Clearing cookies should essentially disable this functionality and require a 2FA code to log in again.

When you sign out of iCloud using the browser (Click your name in the top right corner and Sign Out), you are also presented with this dialog:



If you uncheck Remember me on this browser so I don't have to verify my identity next time. and sign out successfully, the browser will require a 2FA code the next time you log in.