What is the ruling on someone who has frequently backbitten many people and repented, but was unable to seek forgiveness from some of them due to forgetting who they were? And do abundant good deeds and remembrances suffice as an alternative to seeking their forgiveness?

This problem stems from the ill effects of sins related to the rights of people. You must seek forgiveness from those whom you slandered before it is too late. If the person is unaware of your slander, then it is preferable to seek general absolution without mentioning details. You should also multiply good deeds, pray for them, and praise them for their good qualities. As for seeking forgiveness for someone you slandered, there are weak hadiths concerning this, but they have corroborating evidence, such as: "Whoever slanders his brother and then seeks forgiveness for him, it is an expiation for him." It has been attributed to Al-Hasan Al-Basri and Mujahid that merely seeking forgiveness and praising the slandered person is sufficient. It has also been narrated from Ibn Al-Mubarak that seeking forgiveness alone suffices. If the slandered person is absent or deceased, then you should multiply seeking forgiveness for them, praying for them, and performing good deeds on their behalf.