TLDR: DMZ is part of Warzone not MW2. With MW3 we're not getting a new Warzone, but because MW3 we're getting an updated Warzone, and that means DMZ too. FOR FREE!
Warzone's free-to-play DMZ extraction mode will remain available to access through the Warzone section of CoD HQ app. DMZ will continue to be fully playable across all existing maps, including Al Mazrah, Ashika Island, the Koschei Complex, Building 21, and Vondel.