https://www.pcgamer.com/borderlands-3-pc-is-releasing-exclusively-on-the-epic-store-in-september/

Borderlands 3 will release September 13 on the Epic Games Store, which will be the only place to buy it on PC until April 2020. After that six month exclusivity period, it'll release "on additional PC digital storefronts," according to a press release from 2K. 

“Our mission, in addition to delivering the most engaging and captivating entertainment experiences, is to embrace new distribution platforms that will contribute to our effort to grow the audience for Borderlands 3," said 2K President David Ismailer via the press release.