Nuketown 2025 was originally released as a free map with every pre-ordered copy. Having the map meant you earned double XP during the launch weekend. However, it was put out of play when the launch weekend ended.
The original plan was to make it available to play during special events but as we know now, Activision and Treyarch have been convinced otherwise. Here is Treyarch’s twitter post that confirms the return of Nuketown 2025:
Just launched a new fast-action playlist in the bonus category called Chaos Moshpit. Back by popular demand: Nuketown 2025.