Gearbox Software updated Borderlands 2 and Borderlands: The Pre-Sequel recently, forcing a rather annoying SHiFT login on both games, even if you want to enjoy them in singleplayer. The highly likely reason is the upcoming arrival of Borderlands 3 to Steam.
Borderlands 2 is an extremely popular title on Steam, considering that the game is old enough to have spawned two sequels afterwards. Many enjoyed the multiplayer looting and shooting but this could potentially change in the future.
Gearbox Software seem to be moving away from relying on Steamworks for the multiplayer as they are now forcing their own accounts on the existing users. Borderlands 2 and TPS can no longer be accessed at all until the player makes a SHiFT account for the purposes of matchmaking and "cross-play" on PC.
SHiFT account prompt will pop up as soon as the game loads up and there was no known way of bypassing it at the time of writing. The main menu was obscured and could not be accessed at all, meaning that if you wanted to enjoy your game in singleplayer offline mode, you can not do so until you do the SHiFT chores.
Even though creating the new SHiFT name is not a complicated process, this new requirement is essentially DRM that prevents the paying customers from playing the game unless they comply with a new requirement that was forced in, years after the initial purchase. With that reason alone, it would not be hard to understand why the players are outraged.
However, this is not where it stops. The SHiFT update keeps advertising Borderlands 3 and players have to go digging in their game files to get rid of these. It also remains to be seen how good the new service will be when it comes to multiplayer since Gearbox doesn't really have a good track record with these.
There is speculation that the reason for forcing SHiFT is the upcoming release of Borderlands 3 on Steam which is set for 13 March 2020. This game is also available on Epic Games Store and handling multiplayer through SHiFT accounts makes sense in order to make the players able to play with friends who bought it on a different storefront.
However, it hardly excuses messing with Borderlands 2 and TPS to this degree.