Is your feature request related to a problem? Please describe.
You must use Optifine to use internal shaders, which is very annoying. Internal shaders allows you to disable "Old Lighting" which some people like the look of it disabled.

Describe the solution you'd like
To add it to Iris and be able to set it in game

Describe alternatives you've considered
Optifine, but it is so slow compared to Sodium

Additional context
Nothing

Internal shaders is a legacy feature left over from shadersmod, when you had to embed the shaders in the jarfile. All it does with a normal jar is run the vanilla shaders through the optifine pipeline, which had a few side effects like the lack of old lighting.

It makes no sense to implement this in Iris since there's no reason to embed the shaders in the jarfile anymore.

Internal shaders is a legacy feature left over from shadersmod, when you had to embed the shaders in the jarfile. All it does with a normal jar is run the vanilla shaders through the optifine pipeline, which had a few side effects like the lack of old lighting.

It makes no sense to implement this in Iris since there's no reason to embed the shaders in the jarfile anymore.

Some people like the look of "new lighting", as I have said so that is why I am suggesting this.

The answer is no; there are mods like Simply No Shading to do this.

Internal shaders was never added to Iris, and is not supported by Iris.