If you create an app icon with Apple's Icon Composer, even if you explicitly disable the Liquid Glass effect, the icon still renders with an unsightly glaze on the homescreen 1.

To fix this, right click on the .icon file in Finder and Show Package Contents. Open icon.json in something like TextEdit and remove the shadow and translucency properties.

{
  ...,
  "groups" : [
    {
      "layers" : [...],
-      "shadow" : {
-        "kind" : "neutral",
-        "opacity" : 0.5
-      },
-      "translucency" : {
-        "enabled" : true,
-        "value" : 0.5
-      }
    }
  ],
  ...
}

Congratulations, your app icon should now be unglazed.

1

I realized later on that there is a way to turn it off from the GUI – I was just on the wrong layer; there are controls for Mode, Specular, Blur, Translucency and Shadow. Might still be faster to remove the effects via the .json file though.