Does denying it in one's heart and deleting messages while remaining in a group where enormities and prohibitions are sometimes sent absolve a person of sin, knowing that this group is important for study, and leaving it may lead to being deprived of studying?

You do not sin by remaining in the group if you advise them individually, because by doing so, you have denounced their wrongdoing and absolved yourself. The Messenger of Allah, peace and blessings be upon him, said: "If a sin is committed on earth, he who witnesses it and dislikes it — and on one occasion he said: denounces it — is like one who was absent from it. And he who was absent from it and approved of it is like one who witnessed it." Denunciation by the heart is an obligation upon every Muslim in all circumstances, whereas denunciation by hand and tongue is according to one's ability.