Thanks for the heads up. I've investigated this, and found out that the pre-gamma screenshots were taken with the slime alpha set to a different value.
This screenshot was taken back then with the glass fully transparent. Now I've used it to adjust the alpha value for the slime, and found out that
the value was 0.6. The slime alone is a little less greener now due to gamma correction preserving more of the background hue,
which is mostly brown.
With a slime alpha of 0.6 and soft depth disabled,
this is a more accurate screenshot for comparison. By the way, this made me notice that the shape of the slime texture is also preserved a lot better now.
And Retroquad doesn't support fog. I've thought a lot about fog rendering, and still can't confirm if I'll try to implement it.