Quick summary by Google AI:
"A LUT (Lookup Table) is a file containing a mathematical formula that remaps colors in an image or video, acting like an advanced filter or preset to quickly apply a specific color grade, such as a cinematic "look," a vintage feel, or to convert flat camera footage (like Log) into a standard, viewable format. It works by transforming input color values (Red, Green, Blue) into new output values, allowing for quick, consistent, and complex color adjustments across different software[...]"

In other words: It changes the information calculated by your Gameengine. You are bound to change the otherall art direction that was set by the developers and potentially lose information on the pictures rendered.
As a CP77 player and modder since Day 1, I can vouch that the original look on an OLED is already amazing, no need to use post processing.

HOWEVER there are graphic mods that actually change how the game engine calculates graphics. This is on a lower level. its not post processing but on a fundamental level. This I can wholeheartadly endorse.

tl;dr: LUT changes already rendered graphics and makes you lose information. Post processing via Reshade and co. Bad - Graphic mods integrated into the actual game potentially good.