Back when Return to Castle Wolfenstein was still a new game, there were plans to create an expansion to the game that would feature a squad-based single-player component. However, the game, which would be called Wolfenstein: Enemy Territory, wound up forgoing the single-player and became a free-to-play multiplayer-only title. However, a modder named William Faure, also known as Dark Matter Productions, has created a recreation of that single-player campaign with the help of the overhaul mod RealRTCW. The mod makes use of the maps originally in Enemy Territory, but repurposed and redesigned for a story-driven single-player campaign.
On the mod's Mod DB page, Faure talks a bit more about the mod and its creation. The idea for the mod first came up in 2017 when he and KeMoN talked about possibly making an Enemy Territory story-driven map and linking it to a mod Faure made in 2015-2016. That idea was ultimately scrapped, but this, among a few other tests and scrapped projects, led to Faure releasing the first teaser for Enemy Territory Single-Player on May 28, 2019, the 17th anniversary of Wolfenstein: Enemy Territory. From there, a few more teasers were released and the mod was switched over to RealRTCW, which Faure designed the interface for, and released the mod on November 19th 2021, marking the 20th anniversary of Return to Castle Wolfenstein.
You can find and download the Wolfenstein: Enemy Territory single-player campaign here on Steam or on Mod DB. You need Return to Castle Wolfenstein and RealRTCW in order to install it. The Steam version simply requires you to subscribe to the mod, while the Mod DB version has an installer that allows users to choose which maps and addons to install.