There is no event for dispensers that returns both the Projectile (Arrow, Snowball, Potion, etc) and ItemStack associated with it. Each can be individually gotten through BlockDispenseEvent (for the ItemStack) and ProjectileLaunchEvent (for the Projectile) but never both at the same time. This makes some plugins like mine that rely on such things impossible without using NMS.

If a small addition could be made to either event that could add the ability, for example, to associate a shot arrow Projectile from a dispenser and an ItemStack that it originated from I'm sure many plugin developers would be very grateful.

This question from five years ago also covers the issue. https://www.spigotmc.org/threads/blockdispenseentityevent.105512/