In 16.3.3 a new feature has been introduced that allows an administrator to limit access to the Classic UI from the option "Classic PPM Access". If a Channel has been created that points to a Classic UI interface, the user can then use that Channel to navigate to the Classic UI. Furthermore, a new browser tab can be opened, and a user can access the Classic UI directly.
Clarity 16.3.3
If a user opens a Channel to Classic UI, and then navigates to the Classic UI (using a bookmark or changing the URL) then the user will be able to access Classic UI.
This is because opening a channel sets the Classic UI cookie which then allows another browser tab to use the same to access Classic UI.
Additionally, access to Channels that take users to Classic UI is not controlled by this action.