Test material: https://www.dropbox.com/s/986t7r1uy6ds6 ... o.pk3?dl=1
CMF_CHECKTARGETDEAD is a flag for A_SpawnProjectile. From looking at the source and from testing in the game, I found out that the flag makes the calling actor abort its attack and jump to the See state if its target no longer exists, as opposed to doing all of that if said target is dead, as is implied by how the flag is named and what the wiki says about flag itself.
The attached file has a modified version of the Cyberdemon which use A_SpawnProjectile with the flag in question set, instead of A_CyberAttack. Summon it (it's called
Cybor
) and let it fight and kill an imp (remains after death) one time, and a lost soul (disappears after death) another time. In the imp's case, the cyberdemon will continue firing at the imp even after it dies, while in the lost soul's case, it'll abort it's attack after the lost soul disappears after death.