Released in November 2021, No Step Back (NSB) is the seventh major expansion for HOI4. Its name is derived from the infamous Soviet Order No. 227 ("Not a step back!"), which forbade retreat during the Nazi invasion.
It seems you're referencing the release of Hearts of Iron IV: No Step Back . Hearts of Iron IV No Step Back-CODEX
The term "CODEX" refers to a well-known scene group that released a version of the game upon its launch. Note that the group CODEX officially retired in early 2022, shortly after this expansion's release cycle. No Step Back - Hearts of Iron 4 Wiki Released in November 2021, No Step Back (NSB)