Is it obligatory to wash the spot where a thorn was extracted from the foot after performing ghusl for janabah, given that the thorn prevented water from reaching that spot?

It is not obligatory to wash the site of a thorn after removing it if it is not a tangible barrier. What is required is to wash the outer surface of the body, not its interior. If you have washed your entire body with ghusl (definition), the presence of something under the skin does not harm you. The place must be washed to remove impurity if it is not harmed by washing, and this falls under the category of ritual purity from impurities (taharat al-khabath), not ritual impurity (hadath (definition)).