Is it permissible to revile unjust rulers and speak ill of them, and is this considered backbiting?

We have discussed the ruling on reviling rulers. It is permissible to mention an individual who openly commits sins, in the way they have openly done so, for a legitimate religious purpose, such as warning against them or complaining. This is not considered backbiting (gheeba) that is forbidden. Imam Al-Nawawi mentioned that whoever openly commits sin or heresy, it is permissible to mention them by what they have openly declared. It is permissible to backbite an open transgressor, and backbiting is permissible for warning against sin or innovation, for complaining about an oppressor, for a transgressor to someone who can prevent their transgression, for a mufti, and for advice and warning. As for someone who openly commits a sin, they should only be mentioned by that sin, unless they are a scholar whose example is followed, in which case backbiting them is not permissible.