OptiFine 1.8.9 I7+ makes a change which changes sky rendering behaviour. With VBOs enabled in vanilla, the void renders cleanly, not cutting off below the horizon. With VBOs disabled in vanilla, the void cuts off past the horizon line and renders as a deep blue. The change which OptiFine makes results in the sky always rendering as if VBOs are disabled, interfering with custom skyboxes while also looking outright worse. This mod restores vanilla behaviour.
To use this mod, you must be on Forge 1.8.9. You must also have OptiFine 1.8.9 I7 or newer installed.
If you use Patcher, you do not need this mod. This fix is already implemented in Patcher.