Is visiting relatives and friends for Eid greetings a Sunnah, or is that a تخصيص (specific allocation)? And is visiting the living on Eid considered like visiting the deceased?

Visiting relatives, friends, and neighbors during Eid is permissible and strengthens ties while removing ill will. Its permissibility has been indicated by Prophetic traditions, and the righteous predecessors used to do it. It is still among the practices of Muslims to this day.