https://www.pcgamer.com/borderlands-3-loot-xp-boosts-explained/

When Borderlands 3 was properly unveiled last week, some players noticed its pricier Deluxe Edition (as well as its Super Deluxe and Collector's Editions) contained a downloadable bonus called 'Equippable XP & Loot Drop Boost Mods'. I read some speculation about what these boost mods would entail, and I also noticed one player asked Gearbox's Randy Pitchford about it on Twitter, fearing the game's progression system might be built around them, which he firmly refuted:

I decided to reach out to 2K Games and clarify what the Equippable XP & Loot Drop Boost Mods actually do in Borderlands 3. I specifically asked how much of a boost they provide players, and if these editions offer the only way to obtain them. The statement I was issued below doesn't exactly answer the latter question, but the wording suggests to me the item will only be available in these more expensive versions of the game.