Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The scholars defined the beard as being the hair that grows on the cheeks and chin.
Thus, the hair that grows on the neck is not considered as a part of the beard that is forbidden to shave.
Al Nawawi said in Al Majmou’ that Imam Ahmad said: 'It is acceptable to shave from his beard what is underneath the pharynx.' He also mentioned another opinion which is of the view that it is disliked.
Allah knows best.