In my humble opinion, the lower neck is muttar l’chatchilah.
The Mechaber paskens (181:11) that since there are varying opinions in the Rishonim as to the locations of the ??? ????, a ??? ???? should not use a razor on his beard at all. The ??? adds “and even below the throat,” which seemingly answers your query, and not like me!
However, see the Beis Yosef which is the source of this halacha (?”? ???? ?”? ???”?) where he cites R’ Yonah that below the throat does not present a problem of ??? ????. However, he says that one still may not do so because of the issur of ?????? ????.
Take a look in the next siman where the Rema writes that the specific ?????? ???? which are mentioned by chazal such as shaving certain areas of the body are muttar l’chatchilah for men in societies where it is the norm for men to remove the hair in those areas. In ???? ? where the Mechaber says it is assur for a man to look into a mirror R’ Akiva Eiger says that the norm of the non-Jews determines the norm for us in this halacha. If the norm by the goyim is for the men to do it, it is not ?????? ????.
Being that I think everyone will agree that it is the norm in our society for men to shave their lower necks, and for non-Jews to do it with a razor, I think that the second half of R’ Yonah quoted by the Beis Yosef does not apply today, and neither does the ???.
Therefore, ??? ?????? ???? ??????, and it is muttar for a man to use a razor on his lower neck.
Of course this is just ??????? ????? as I am not qualified to pasken.