Must someone who has slandered others and repented seek their forgiveness directly, or is it permissible to seek forgiveness through an anonymous message that does not reveal their identity?

Backbiting is a major sin. Repentance from it involves regretting the act, seeking forgiveness from Allah, and seeking absolution from the person who was backbited.

If the person who was backbited is unaware of it, it is better not to inform them; as that might cause them harm and distress. The repentant person should then suffice with seeking forgiveness for them and mentioning them positively in the situations where they were previously mentioned negatively.

The majority of scholars hold that it is not a condition to inform the backbited person about what happened nor to seek their absolution. It was narrated from Anas ibn Malik (may Allah be pleased with him) that, "The expiation for one who has backbited is to seek forgiveness for them." Furthermore, informing them might lead to enmity and hatred, which goes against the Lawgiver's intent of uniting hearts.

The door to repentance is open, and Allah rejoices in His servant's repentance. His mercy is vast; He forgives all sins.