Gearbox and 2K signed a shorter exclusivity deal with Epic Games for Borderlands 3 as it was exclusively on that store for six months. That period is coming to an end on 13 March 2020 when the Steam version will go up.
Epic Games and Valve's storefront wars have split the gaming community as people found reasons to support one of the companies, none or both. Controversies happened and fast forward to early 2020, Epic Games is showing no signs of relenting in the exclusivity approach while Steam fans tend to wait out the exclusivity periods and wait for games to be released on their preferred platform.
The waiting period for Borderlands 3 is coming to an end as the game released on 13 September 2019. 2K and Gearbox Software signed an exclusivity deal with Epic Games Store that would last for six months and they are running out in exactly two weeks from the time of writing.
Players can already add Borderlands 3 to their wishlists on Steam but cannot purchase the title just yet. Considering it has had several months of stay at Epic Games Store, it is possible that it will be discounted at launch on Steam. Keep in mind that this is a guess and not officially confirmed by Gearbox or 2K just yet.
The discount at launch is not impossible for several reasons, first among them being that it is no longer a brand new game and a chunky sale is a good way to get to Steam's front page.
Furthermore, 4A Games pulled something similar with Metro Exodus when it launched on Steam not so long ago. The game was initially available for 40 per cent off but it was also gone from Valve's store for an entire year.