There seems to be a little confusion over how exotic weapons drop. To clarify:
Exotic weapons will only be contained in lootboxes that are dropped or bought from the store during an event.
If you are hoarding boxes they will only ever contain exotics that were part of the event at the time the box was dropped. If you are traded lootboxes the same rules also apply.
Here’s when all the exotics at the time of writing are active:
January-February: Big Iron, Leeching trigger, Big Bertha, Cupid bullets
April-June: Buckshot ammo Converter, Laser module, Stress Pump, Plan C, tmtrainer module
July 17th-August: Everything!
Autumn still pending exotics
December: Tranquilizer Bullets, Super Bullets, Another Crowbar