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X absolutely supports depths of 1 and 4 bits per pixel, along with 8bpp, with its VGA server: https://www.xfree86.org/4.8.0/vga.4.html

Thanks for the correction.

I hadn't thought of 1bpp because it isn't interesting on VGA hardware. But you're absolutely right about 4bpp having been a thing.

X does, modern X apps may not.

I remember that Xfree86 allowed 1 bit (B&W) modes