The shaderpack failed to load! Please report the error to the shader developer.
dh_terrain.vsh: dh_terrain.vsh: 0(138) : error C1503: undefined variable "dhRenderDistance"
Game Version: 1.21 (fabric)
The shaderpack doesn't load with the combination of the newest version of ImmediatelyFast (1.3.1) and Distant Horizons.
It works with ImmediatelyFast (1.3.0) tho.
Not asking for support. Just to inform.

I can't reproduce the issue on Minecraft version 1.21 or 1.21.1, with Iris, even with ImmediatelyFast 1.3.1 installed. I understand you're not asking for support, but check what the real cause might be. Make sure you're using the original Iris and not a derivative product.

In fact, Distant Horizons doesn’t work correctly with the shader in version 1.21. (At least not for me, with the latest versions)

I installed Iris with the combination of DH through the Iris discord then just updated the two mods when there was an update.
I have different performance mods and also the Physics Mod Pro installed. I turned of the water reflection for more FPS and the shader works great: