From their FB page:
Upgrade your 🍧 drip 🍧 with this SHiFT code for new Popsychle cosmetics in Borderlands 3!
KH5J3-9BSSS-K6WF9-RT3BB-FTBCJ
To celebrate Borderlands 3's 2nd anniversary, here's a diamond ke — wait, a 💎DIAMOND KEY?!💎
CHC3J-TCHSZ-5XCXH-FTTJJ-JZ55S
Redeem in-game or at http://shift.gearbox.com! Code expires Sept 30.
🎉 Happy #Borderlands3 anniversary! 🎉
Bonus 3 golden keys: WZWJB-CTJSS-5FCXZ-XBJ3T-WTHZZ
Anyone have any idea about what the diamond key is about? How do you use it?