It would be useful to be able to disable client light updates in order to increase FPS while running certain redstone contraptions or large flying machine structures.

You are in luck! This tweak does exist in the 1.12 branch of the code and when Masa brings forward the changes into 1.13+ it will be available to am whatever version you want!

This isnt in the newest versions since mojang did a full rewrite of the rendering engine.

Oh so does the feature need to be rewritten because of rendering engine change?

This was actually removed in the 1.14+ version instead of later added in the 1.12 version, the reason being the rewritten lighting engine in 1.14. I am probably going to add it back at some point, but yeah it needs to be adapted for the new lighting engine, the old mixins from 1.12 and 1.13 won't work in 1.14+ for this.