Witch Doctor's death ward, and the projectile from the deathward also isn't slowed.
I'm not entirely sure if it's a bug, but based on description I think all projectiles from any units should be slowed, it should include those that doesn't come from creeps or heroes or creep heroes too.
Ability name
Time Zone
Description
I've tested a few heroes on Demo:
Clinkz with burning army and the projectiles from the skeleton's attack doesn't seem to be slowed.
Clinkz's Burning Barrage projectile isn't slowed either
ShadowShaman's serpent wards attack projectile aren't slowed either.
Witch Doctor's death ward, and the projectile from the deathward also isn't slowed.
I'm not entirely sure if it's a bug, but based on description I think all projectiles from any units should be slowed, it should include those that doesn't come from creeps or heroes or creep heroes too.
Example Match ID (and possibly Timestamp)
demo test