The only visual cue that exists to identify non-public tasks is the little icon and legend in the header saying anything other than "{icon globe} Public", on dark grey over light grey. In our experience at #Wikimedia, this is not enough.
More often than desired, users are not realizing that they are viewing or commenting in a non-public task. This, in turn, increases the risk of publishing links to private tasks in public pages (other Phabricator, tasks, email exchanges...) including the title of the task and even some comments of the content discussed. This is not a risk for newbies only, it happen to myself as well, jumping from task to task, updating here and there.
This risk will increase when anybody can obtain access to a private task just by being CCed. In fact, the chances are pretty high for busy people with many ongoing discussions in tasks, unless the content of the private task is clearly something that shouldn't be discussed (which is not always the case).
A good solution simple to implement would be changing the background color to i.e. light red in any non-public task (!Public or !AllUsers).
Original report: https://phabricator.wikimedia.org/T76929