You shade it without relocating it, causing QuiltMC/quilt-loader#315.
You also crucially need to mergeServiceFiles or it will break on modern forge.
You also initialize it in a ClientModInitializer, that's way too late for most stuff. You don't seem to initialize it at all on forge. You need to do it in an IMixinConfigPlugin.

Ah, wasn't made aware that it needs to be relocated. Will look into it next week(I'm off to a festival, so not at home).

Yea, I know the pain of people not shading from a different project, so much so that I added some code that will yell at the developer in case it's not setup correctly 😅.

The whole setup process is detailed in the readme.

Decided to use the include way, since I'm not planning a Forge version any time soon.

You're still initializing it far too late.

You might want to rework the readme a bit. I just read

The final step after setting up your build script is to initialize MixinExtras. To do this, simply call MixinExtrasBootstrap.init(); somewhere suitably early in your program.

And took this as "as early as possible"(which would be onInitializeClient in my case). But then the next sentence says that it basically has to be PreLaunch(Something you usually never have to touch/the Fabric documentation recommends against).

As early as possible is preLaunch. Main entrypoint are fired after much of the game is already initialized and so after lots of mixins are already applied.

Yes, but the wording makes it seem like the Client/Common entry points are fine(and it seems to work, at least initially). Most devs probably never had to touch the preLaunch point/are even aware that it's there.

I think it's fine given I specifically say to use it. Regardless, you don't need to initialize it yourself anymore from the latest betas onwards.