The suicide exit is a speedrunning technique in which the player deliberately kills off their character to exit a level.
When killed by a weapon or by blast damage, an actor acquires momentum. Due to the counterintuitive motion exhibited by corpses in vanilla Doom, corpses can trigger walkover linedef types such as teleporters, crushers, and "trapdoor" exits. When the player is killed, their height is immediately reduced to the point that they are capable of sliding under walls or through windows that are normally impossible to circumvent that are at least 14 units in sector height. By combining these attributes, a speedrunner can perform a shortcut by killing themselves, via doing so with the rocket launcher while running backwards, or by allowing enemies to kill them while moving, resulting in them sliding through or under these obstacles and passing over the exit linedef, finishing the level early. This is sometimes called a death slide. Should they continue the game, the player will begin the next level with 100% health, 0% armor, a pistol, and 50 bullets, as if starting a new game.
This technique was discovered in 2001 by Vincent Catalaá on The Plutonia Experiment MAP17: Compound, and subsequently recorded in his 00:09 UV speed demo of the level (pl17-009.zip). In this demo, it is used with the rocket launcher to pass underneath bars that otherwise must be raised by a yellow key switch to finish the level. For this demo, Vincent was nominated for and won the Golden Cyber for Best New Trick at the DOOM Academy Awards 2001.
On occasion, this is combined with other techniques to achieve the desired result. For example, Graham Burgess (Grazza)'s 00:11 UV speed demo for Kama Sutra MAP13: The Holocaust II, available in the Kama Sutra Demo Pack, features a suicide exit facilitated by an Arch-Vile jump in mid-air in order to better fit through the gap, and trigger the exit. In an even more complex example, the shortcut featured in Marijo Sedlić (Sedlo)'s 00:06 UV speed demo of The Plutonia Experiment MAP12: Speed  uses a combination of a rocket jump, an Arch-Vile jump, and a suicide exit to reach the exit teleporter.
Notably, suicide exits can be performed on many of Icarus: Alien Vanguard's levels, as they share exit rooms that feature "trapdoor" exits blocked by barriers that are 32 units in sector height.
Forced suicide exit
In what is often referred to as a death exit, some maps deliberately require the player to die at the conclusion of the level, usually in order to force him to begin the next map from the pistol start. For instance, Scythe 2 MAP25: Forgotten City requires the player to perform a suicide exit to finish the level, while Scythe MAP10: The Lords forces a suicide exit: the player will telefrag an explosive barrel, which will blow up many more barrels nearby. Both the player and Romero's head will die of the explosions, and the level will end.
A mandatory suicide exit also occurs at the end of MAP05: Forgotten Village in Vanguard, thus preparing the setup for the next map, MAP06: Punchline, which is based around fighting enemies with nothing but the pistol and the fists.
Ghostbusters Doom uses the technique at the end of map 03, "Streets of New York", where the player is given a car to drive around and attack enemies (a modified chainsaw) - the suicide exit is necessary to take the car away from the player.