https://www.pcgamer.com/borderlands-3-release-date-leak/

An errant tweet from the official Borderlands Twitter account may have unintentionally revealed that Borderlands 3 will be out on September 13. The tweet, which appeared to be an ad that fired too early, has since been deleted, but luckily we managed to capture an image, and it's also been uploaded to the Internet Archive.

Gearbox has used the word "mayhem" to describe Borderlands 3 several times previously, so while the title itself isn't in the tweet, it's pretty clear what the reference is.