The latest patch just fixes an extremely rare and minor glitch involving boss doors that only a speed runner with hundreds of hours of practice is likely to ever find.
Apologies to everyone confused about why there was suddenly an update and especially to any who have slow download speeds. Given that it has been nearly a year since the last patch and this being such a tiny update, it seems unlikely there will be any significant patches after this.
To see the full history of patch notes, check here: http://steamcommunity.com/app/247240/discussions/0/846965056681025177/
Changed files in this update