A new cheat code for the Sega Saturn version of Doom has been discovered, 27 years after the game’s original release in 1997. This newly found cheat allows players to make walls in the game semi-transparent, offering a unique gameplay feature that was previously unknown.
As an old school doom mapper from the 90s, it absolutely isn’t. You build a room, apply F_SKY1 to the ceiling, and that’s it. The engine renders it more like a wall, but it’s not a traditional skybox in that it doesn’t exist in the level geometry outside of the room it’s applied to.
https://doomwiki.org/wiki/Sky