Ahead of release, Nixxes said that DirectX 12 support for Deus Ex: Mankind Divided would be delayed until the week beginning 5 September. That’s the week we’re in now, and it does indeed seem as if DX12 support is coming tomorrow.
The source on that is an Eidos Montreal community manager posting on Reddit. In response to a query about DirectX 12 support, they say “tomorrow!” (8 September). At the time of writing there’s no mention of this on the Deus Ex: Mankind Divided Steam hub, but the same community manager gave accurate information about a (now released) fix for the late-game subway travel crash bug, so they seem reliable.
DirectX 12 tends to favour AMD cards, so unless this game bucks the trend that’s likely to be the case again. Nixxes released several DX12-related patches and updates for Rise of the Tomb Raider, so tomorrow’s release probably won’t be the one and only iteration.