I use complimentary reimagined, and the piglins are invisible, can only see swords.

For future reference, this is why Spirit doesn't work with Iris shaders, and probably never will: https://github.com/IrisShaders/Iris/blob/a71b6893c7b8a3d456e121da1794f2e87ec5dcd2/docs/development/compatibility/core-shaders.md

It would be nice to have the option to toggle this mods spirit shader effect, so that if you use a shader then the mobs won't appear invisible

You can remove the custom spirit shader by removing this part from spirit.common.mixins.json:

  "client": [
    "EntityRendererMixin"
  ],

You can do so by opening the .jar with something like 7zip, copying the file, editing it and then putting it into the openend 7zip browser of the .jar again (= overwriting the file)