The Space Suit is a scrapped Duke Nukem 3D item.
It was an item that would allow Duke to survive in the vacuum of space, the way that the Scuba Gear allows Duke to survive underwater. It was ultimately removed, and the only time Duke actually ventures outside a space environment (in the level Dark Side, where he walks on the surface of the Moon), some kind of shield is in place allowing for a breathable atmosphere (a Ceiling Palette different from 0 bypasses death effect). Nevertheless, the sprite for the item still appears in the released game, in Dark Side, albeit as just a decoration. It also features in the Zoo demonstration level next to the other items, indicating that it was originally meant to be a usable item in the game.
There is a lot of unused data pertaining to the Space Suit: a status bar icon for the Space Suit, messages that would be displayed (e.g. "SPACE SUIT ON"), and the tiles "2584/SPACEMASK" (Space Suit outfit) and "2578/GLAIR" (transparent glass) that would show the Space Suit's visor as it would appear when worn (see the image on the right).
In the pre-release version 1.0, the Space Suit exists as a proper item. It can be picked up, and it also works as intended (i.e. it prevents instant death when exposed to the vacuum of space). It also has some other effects, such as reducing the player's speed and increasing their jump height, to mimic conditions on the Moon (thus implying the lunar facilities in Lunar Apocalypse have artificial gravity, rather than simply handwaving the otherwise unrealistic gravity in that episode away.
The Space Suit functioned just like the Scuba Gear: they both automatically activate when in a particular region (vacuum/underwater) and slowly drain air.
The Space Suit item was removed from the game later in development, and the level Dark Side was altered slightly such that the player could survive outside. However, as mentioned above, the Space Suit sprite and airlock were not removed from the level design.
But it was later used in the original form in the Genesis version of Duke Nukem 3D