The scout appears in the single-player campaigns, universally operated by AI opponents. They cannot be built by the player.
Legacy of the Void
Compared to the Wings of Liberty and Heart of the Swarm campaigns, the scout is more fragile and has a weaker anti-air attack, but has a much stronger ground attack. Though they cannot be built, Dark Archons can take permanent control of enemy scouts.
Fenix can build purifier scouts in Co-op Missions. Fenix can upgrade a scout into Mojo, turning the scout into a hero unit that can attack in an area of effect and stun groups of targets.
The scout can be utilized by Amon's forces. A scout is a possible gift that can be gained during the "Gift Exchange" mutator.
Combat Sensor Array
Scouts and Mojo gain +3 range against air and +1 range against ground.
The Scout's design for Wings of Liberty went through several phases, as while its new design was to have homages to its original form, designers had different features they liked about the original. Another concern was making it look more organic in accordance with the protoss units in StarCraft II having more flowing shapes and fewer geometric angles. A design was finally settled on, with the StarCraft-esque wings being replaced by more organic looking ones.
Incomplete tooltips for the scout imply it may have been intended to be usable by the player in the StarCraft II: Legacy of the Void single-player campaign. These tooltips imply the scout would have been a purifier faction unit, and have an Afterburners ability that boosts its speed.
Dark scout model (Wings of Liberty and Heart of the Swarm single-player campaigns)