In Destiny 2, Exotic weaponry is usually supposed to represent the pinnacle of its niche. Whereas Legendary weapons generally come with fewer perks that might not synergize quite as well as they do with Exotics, Legendaries do get the added bonus of being able to support mods, something that Exotic weapons won't be doing according to Proctor.
Bungie's Staff Designer Chris Proctor has gone on record to talk about some of the changes that are coming to Destiny 2 with the release of its Witch Queen DLC. One of the major bits of information revealed was the fact that Exotic weaponry would never get a mod slot, which may be a source of disappointment for some players.
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The matter of allowing mod slots on Exotics was brought about as part of a recent podcast that Proctor took part in. When asked why Exotic weapons don't have a mod slot, he explained that opening up Exotics for modding would introduce game-breaking glitches to Destiny 2. The specific examples he provided were that of Icarus Grip attached to Ace of Spades, which would massively increase the hand cannon's midair accuracy. Similarly, putting Backup Mag on Hawkmoon would potentially destroy its damage-stacking Exotic perk.
To that end, it would seem that modding will stay in the purview of Destiny 2's Legendary-tier weapons, which is being bolstered even further with the release of a gun crafting system in Witch Queen DLC. According to Proctor, Exotics are designed with a more specific purpose in mind than Legendaries, and since they come with curated perks that are often more powerful than the ones available on Legendaries, so adding mods simply isn't on the table.
Of course, it's worth keeping in mind that
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