Historic Legality is pretty arbitrary, but the recent addition of it to Companion's card database features a number of missing cards, missing sets and errors. In order, after a cursory check:
-Jumpstart: Historic Horizons is completely missing, which is problematic since it features some of the strongest and most played cards in Historic, from Dragon's Rage Channeler to Thought Monitor.
-Partly related, digitally exclusive cards are missing from Companion, be them from Jumpstart: Historic Horizons or later Alchemy sets, yet they're legal in Historic. Probably should be around if it's a supported format. (But then probably would have to have a way to display digitally rebalanced cards.)
-The banlist is simply not accurate. Memory Lapse shows up as legal when it has been banned for months. Omnath, Locus of Creation and Teferi, Time Raveler show up as banned when they've been digitally rebalanced and unbanned (although Omnath is still on the official banlist on Wizard's site as of yet, he is playable in Historic.)
-Cards added to Historic with Brawl events aren't shown as legal in Historic. Off the top of my head, those are The Gitrog Monster, Bladewing the Risen and Rhys the Redeemed, though I may be forgetting some others.