Sorry, I am probably ignoring something, but in one story mission on Mercury in D2, one of the first ones, you find weird Vex who are bronze and look weird(I dont remember the names, but they are not Precursor.) If this is their only instance, then why even use them?)
They are Descendant Vex, Vex from the future. Curse of Osiris wasn’t the only time they were used. Descendants were seen in the Vault of Glass and in the Undying Mind strike.
Don’t worry, when Vault of Glass gets brought back into Destiny 2, you’ll be seeing those Vex a LOT more. Trust me, the final boss and its mechanic rooms will be brimming with Vex of Precursor, Present, and Descendant variety.