One of the all-time great role-playing game strategies is to grind away at earlier content -- whether through side missions, completely exploring every area, or killing absolutely every enemy to maximize experience -- so that upcoming stages are easier. The concept of being over-leveled isn't new but Gearbox doesn't want Borderlandsplayers unchallenged anymore.
As part of this week's round of hotfixes, Gearbox is rebalancing some of Borderlands 3's later areas so that enemies are brought up to the player's level. This specifically applies to missions on Nekrotafeyo and The Anvil. Gearbox reasons that being overly capable "trivializes those areas, as enemies were sometimes much weaker than intended for that point in the game."
However, existing characters don't need to be concerned about this reconfiguration. Gearbox mentions that it will only affect new characters, meaning anyone you've already created won't be subject to this change. Every character created after the hotfix, though -- and that includes all the ones that will be made tomorrow when Borderlands 3 finally lands on Steam -- won't be able to dodge the balance changes.
Here's the entire patch notes for Borderlands 3's March 12 hotfix: