This page of the Wolfenstein: Youngblood game guide informs you where to find the easter egg that will allow you to play the classic version of the series – Wolfenstein 3D.
You can play Wolfenstein 3D after reaching the catacombs or the hideout of the local resistance movement. The catacombs appear for the first time after completing the second mission (Riverside), but you can also return to them any number of times later.
Go to the hideout bar where there are two slot machines. You need to interact with any of them.
The game zooms in – parts of the car will still be visible at the edges of the screen. Wolfenstein 3D is fully playable. You can select the episode you are interested in, as well as view the control and determine the difficulty level. Wolfenstein 3D also lets you save game state so you don’t have to play through everything at once.