Is it obligatory to inform those who were backbitten or spied upon and repent from those evil deeds, given the doubt about reaching puberty at the time of committing them due to the presence of coarse pubic hair then?

The growth of coarse hair around the pubic area (al-inbat) is a sign of puberty, and this is the view of the Malikis and Hanbalis. Therefore, if you committed sins after the appearance of this coarse hair, you were an adult and must repent.

If the sins concern the rights of individuals (such as backbiting and spying), the most preponderant and strongest opinion is that it is not obligatory to inform the person you backbit or spied on. Rather, it suffices to supplicate for them, seek forgiveness, mention them positively, and defend them from backbiting by highlighting their good qualities in the presence of those to whom you backbit them.

As for old, unknown sins that you committed during your adulthood, general repentance for them is sufficient, without the need to recall each sin individually.