For some odd reason, installing Reshade on Borderlands 2 completely removes the dynamic shadows for the game, resulting in a mostly fullbright experience.
This happens simply with Reshade being installed, effects are disabled in the second screenshot for consistency so its easier to tell the difference.
Vanilla:
Reshade Enabled:
This seems to be an issue with DX9 specifically, as running the game under DXVK does not have this issue.
I unfortunately do not have any older versions on my PC to test, but I do not remember this happening on older versions, so a good place to start testing would probably be 4.9.1, in case it is a new issue with 5.0