If you compile #foot master branch against the new pixman 0.46 release (and check meson isn't using an old, cached, version number for pixman) you'll get support for 16-bit surfaces. These are now the default when you enable gamma-correct blending.
As mentioned before, both 10- and 16-bit surfaces are slower than 8-bit surfaces, and if you want to use gamma-correct blending and don't mind less accurate colors, you can force 8-bit surfaces by setting tweak.surface-bit-depth=8-bit.