I don't think that this can be done from QL directly. Looks more like setting a very low max size for textures from the GFX drivers. Some folks achieved simular results with NVidea drivers for QC IIRC.

you sir are a legend, literally figured it out once you mentioned nvidia inspector. For anyone in future searching, heres how to do it.

Antialiasing - Transparency Supersampling
0x00000008 AA_MODE_REPLAY_MODE_ALL

Texture Filtering - LOD Bias 0x00000078


However i still can't get the shadows to blocky so we still gotta figure that out

You can always bet on ESR for having obscure knowledge about quake graphical tweaks.

Could this be considered cheating since it's using a 3rd party software to change the appearance of the game?

Ofc, I'm sure even a simplest anticheat will ban you for trying to use nvidia inspector or anything similiar. Only in quake this even comes up as a question with ppl being used to "config arms race"...

I seem a guy on Twitch the other day streaming QC PTS and it didn't look too far off that.

Looked like picmip 10-16 kind of cool to see in QC actualky.