Is it necessary to seek forgiveness from someone who was slandered if years have passed since the incident and the relationship between the two parties has returned to normal?

Since the matter has passed for some time and the person has forgotten it, there is no need to remind him of it, because that stirs up animosity and resentment, and its harm outweighs its benefit. Seeking forgiveness and supplicating for him, and remembering him with good words, are sufficient.