I assume that Starfleet probably told them about shit that happened since they left. But Trekkies overreacting to shit is hilarious.
Vance didn't debrief Discovery on Ni'Var and how Romulans and Vulcans were the same species until they needed to go there and Burnham was used as a way to get SB-19 data.
Burnham displayed a shocked look when Cance said Vulcans and Romulans were the same species.
Its possible that Discovery got debriefed on some points of the past, but as it was pointed out, they did acquire the updated Federation database which covered everything since their point of departure to the 32nd century... so its also possible that Disco crew was told to familiarize themselves with the history of the past 931 years in their own time... or as missions require it (we keep seeing that the crew learns new things on a mission by mission basis, so, its likely no official debrief on the past 900 odd years was given as the info would be in their databases).
Plus, Discovery has pressing things to worry about atm (such as the Emerald Chain and the cause of the Burn). Although it would be good to see the crew take some initiative and mention at some point they've been studying past historical events since arriving in the future.