If you wanted to add shadow on Polybar, you can add this config to your picom.conf

i don't use compton or another compositor.

wintypes:
{
  tooltip = { shadow = true; opacity = 0.75; full-shadow = false; };
  dock = { shadow = true; full-shadow = false; }
};

and also, set no-dock-shadow to true on picom.conf

no-dock-shadow = true;

If you want to disable shadow on Polybar, just simply make them to false

wintypes:
{
  tooltip = { shadow = false; opacity = 0.75; full-shadow = false; };
  dock = { shadow = false; full-shadow = false; }
};

no-dock-shadow = false;