You Don’t Need To Have Prior Experience With The Series
First and foremost, it’s important to note that you don’t need to have any prior experience with the Deus Ex series before playing Mankind Divided. The game takes place nearly 20 years before the original Deus Ex that released back in 2000. It is the sequel to Human Revolution, however Eidos Montreal has noted that they want Mankind Divided to be accessible to new players as well, by telling a fresh story in a new setting.
This isn’t to say that there won’t be some links to Human Revolution, as Mankind Divided does have a lengthy section catching players up on the story of the previous game. You’ll merely have more context for the story that the new game tells if you’ve played Human Revolution.