As a dev I always hope that the user's monitor is calibrated. The situation is definitely better than it was 20 years ago, with a lot more monitors coming from the factory with better defaults, especially Apple devices, but you really need a color calibration widget to do it properly, e.g.
No proper calibration, but the closest I've seen is in the Myst series. According to my fuzzy memory, either Riven or Exile (or both) had a setup screen with shades of gray, it instructed you to adjust the brightness / contrast of your monitor until the shades looked a certain way. My search-fu is failing me on this, so maybe it's a fabricated memory.
You'd do that at the OS level, games don't need to re-invent that. Games do now ship with HDR calibration wizards to deal with the particular aspect of luminance range and SDR white point being totally undefined and inconsistent, at least for almost anything connected via HDMI or DisplayPort