The 2.5GB patch released for Deus Ex: Mankind Divided on PC today is a Breach mode centric update, and doesn’t seem to address much else. In the words of the accompanying news post on Steam, it “focuses on improvements for [Breach].”
That’s basically it in the way of changelog or update notes. If, for some reason, you want to pop back to PC patch 12, it’s possible to do that through the Deus Ex: Mankind Divided beta tab (right-click on the game in your Steam library, select Properties, then the Beta tab, then the drop-down menu).
The game’s second story-based DLC, A Criminal Past, is due for release in “early” 2017. Update: In fact it now has a release date: 23 February. It’s quite possible that some of the 2.5GB is data for the DLC.