Borderlands 3 will have cross-platform play at some point, but it won't be available at launch. This is a feature that Gearbox has repeatedly called a "pre-requisite" for a modern day Borderlands experience, so the fact that it won't be available when the game debuts on September 13 is a disappointment for some, but Gearbox has tried to assuage those concerns by promising that it's coming. We're just not sure when.
Cross-platform play has been called for in Borderlands 3 since the game's original announcement and it's only been growing as we drew closer to the game's release. Gearbox has shown a key interest in the feature, suggesting that that was the reason it was sticking to the Epic Game Store only, rather than making itself available on Steam. It suggested that with Epic's strong commitment to cross-platform play with Fortnite, that it was a great fit for Borderlands 3 too.
As Eurogamer points out, the fact that publisher 2K Games pushed the idea of "togetherness" in Borderlands 3 seemed to suggest that cross-platform play was a major focus for the companies working on the game. But it has since said that instead of being available in mid-September, that the feature would be cokming "as soon as practical."
There is a big reveal taking place later today, that's all about "togetherness" too, but what that means is anyone's guess. You can watch the Steam live on YouTube at 7AM PT, or in the embedded video below.