I think this documentation may actually be outdated/incorrect. It would be easy enough to create a subclass of MagicProjectile which destroys itself (see the code in MagicProjectile.ReturnToObjectPool) after X seconds though.
Update: in version 2.3 the MagicProjectile class has been changed, so that now it implements the Projectile class and has the lifespan parameter. So you may be able to update to 2.3 and just use that instead if needed.
Thank you for this information. I have updated my package but MagicProjectile is still implementing the TrajectoryObject class. Maybe this is because I am using the Third Person Controller and not the UCC? Did you also update the TPC?