Is it permissible to receive Islamic knowledge from a shaykh via social media platforms without the knowledge of parents who object to it?

There is no harm in seeking knowledge from a shaykh via social media in public groups. Your parents do not have the right to prevent you from doing so without a legitimate Shar'i reason, for seeking knowledge includes that which is an individual obligation (fard ayn) and that which is a communal obligation (fard kifayah). Obedience to parents is conditioned on what is beneficial and not harmful to the child. It is obligatory to avoid seeking knowledge through private messages, as it is a potential source of temptation (fitnah).