Is the failure to visit a negligent father who abandoned his children for 20 years considered an act of disobedience to parents (ʿuqūq) that prevents entry into Paradise? And what is the sin of the father in this situation?

A father is not permitted to neglect his children or squander the trust placed in him, for every guardian is responsible for his flock. However, a father's neglect does not justify his children abandoning him, boycotting him, or not visiting him. Disrespecting parents (Uquq al-Walidayn) and severing ties of kinship (Qati'at ar-Rahim) are among the grave sins that prevent entry into Paradise. The Prophet, peace and blessings be upon him, warned against disrespecting parents and severing ties of kinship. Therefore, children must maintain ties with their father, show him devotion, and treat him with kindness.