ZDoom

By default, yes, because it's designed to make Doom... DarkZ

However, check under Advanced Options and you'll see options named Pre-Gain and Post-Gain, and Min. Light Level. The first two apply separate independent adjustments before and after the contrast darkening curve, and the last one clamps the final light level to whatever minimum you set.

So for example, if you want to simply brighten the level, put your mode/preset on Disable and set pre-gain to +32 or something. If you want to adjust the contrast from there, play around with the Compress or Crush modes and other gain adjustments. If you like the overall level lighting but there's just a few bits that are WAY too dark, raise the minimum clamp to something higher.

I tried DarkDoomZ and it "squishes" brightness, lessening the contrast between bright and dark areas. Or maybe Im setting it up wrong, in that case I'd like to have a working config that bumps the brightness of every sector by a specified amount (~22 or so) but keeping the contrast.

DarkDoomZ has a fairly robust light curve adjustment system now (including brightening), take a look and see if that meets your needs?

I don't know if it's possible, but I had the idea of fading out the shading multiplicatively, ie if you set it to "0.5" then you get halfway between a properly shaded scene, and fullbright. Which would be quite enough to really cheat most wads, but you could set it to a lower value if you wanted...

That old option clamped the bottom light level and proved to be a genuine hassle for mappers because it interfered with map design - and it won't come back.
But I have no idea if the requested feature is really the same.

Ah, yeah, I forgot about that, that did exist once.

I'm fairly sure that used to be an option (Ambient light level or something), and it was removed some time ago AFAIK specifically because it caused problems for mappers, since it meant there was no way to make something look the same for everyone.

What you're suggesting is essentially a cheat, in my opinion. Which I have no real problem with, but that's not something that many folks are going to be happy with unless it's locked up behind a cheat-blocked CVAR or command.

But I do think this is a good idea, regardless, there's been many times when I've wanted to see through the darkness in a map without using the light-amp goggles.

This one proves a bit more problematic to explain.

Because it's based on something I have seen in action but... didn't understand well enough to properly explain. It would be a second type of "gamma correction" of sorts, but one that actually affects the environmental brightness of the maps. I think it's time for something else to step in and more-or-less replace the old gamma correction, which makes everything look horribly washed out when increased.

For an example of this in action I'd suggest checking out GEC GZDoom Master Edition. As I said, I have no idea how exactly was this achieved in that port (let alone if the implementation was optimal), but it's kind of what the "D64 Brightness" option under Display appears to do, and it's so much nicer than the old fashioned gamma correction.