Just out of curiosity, do you instantly get Necromancy when you make a demonic pact? Or is that a separate thing?
A warlock with a demonic pact is almost like a twisted inversion of a priest, but instead of their god granting them power, it's a demon lord. Since their magic comes to them via the demon, nearly all of their magic is demonic in nature. So, yes, they all have access to necromancy. Whether or not they can actually perform powerful necromantic spells depends on the individual warlock's knowledge, skill level, and how many souls they are able to sacrifice, since more powerful spells require a higher price.