It’s not unusual for Google to add and remove elements during the examine program leading up to a new OPERATING SYSTEM release. It happens continuously and is exactly what we’ve noticed with while checking out Developer Preview 3 or more. The System UI Tuner is really a place for experimental features anyway, so anything discovered there is always subject to change. The most recent Android beta sees removing all 3 of the most current additions to this menu.
Android O function spotlight: System UI Tuner loses features in Developer Preview 3 was authored by the awesome team at Android Police.