It's standard to have a brightness slider in games. This isn't competitive, so no one is getting an unfair advantage by adjusting the brightness to be comfortable so I can't see why it wouldn't exist. Windows 11 removed the slider for brightness for a lot of people, including myself. And I can't seem to adjust it in the displays manual settings any more. I've also heard not having a brightness adjustment is terrible for people with HDR displays(they complain about is for Binding of Isaac as it only has a gamma adjustment). It's a standard QOL setting.

(the mods on steam said to post here for suggestions)