So this is purely a rumor, technically, but when you see the source of it, it'll be a pretty strong one.
An accidental tweet from the official Borderlands Twitter account showed that "Mayhem" is coming "September 13th", a very strong indicator that the game will be releasing in September. It's likely a pre-scheduled tweet that had the wrong date put in, but thankfully PC Gamer quickly grabbed a snap of the tweet before it went away.
It makes sense, as Take-Two have previously said that Fall 2019 was their intended release window. Of course, the issue is the tweet also went out on April 1st. A bad April Fools joke, perhaps? It seems unlikely, given how damaging such a tweet is, but you can never rule it out.
Twitter user Wario64 also noted that the game will also be released on the Epic Store, although there was no mention of an exclusivity deal there. A few seconds of footage was leaked, in the tweet below, where this information was obtained.
Borderlands 3 releasing on Epic Games Store (from a Twitter ad video that is now deleted) pic.twitter.com/d4v17GapWZ— Wario64 (@Wario64) April 1, 2019
So, yeah, if it was a mistake, it's a pretty bad one.
Stay tuned to GameFront for more news on Borderlands 3.