It's probably legal for the 'shareware episode' assets (same for the Doom shareware version), although I guess it's better to let a lawyer dissect the license just to be sure:

https://www.quakeworld.nu/wiki/Quake_Shareware_License

This seems like a good use of AI. Get some random good enough textures to bundle. Give me a lake of pixelated radioactive acid.

Or just use a free as in freedom Quake data, such as Blasphemer.

Assets could be possibly downloaded from 3rd party site on user consent.

An app that doesn't work until you use it to pirate something? Yeah that wouldn't fly in App review.

I understand the intricacies of it in terms of the App Store, but I don’t think grabbing Quake’s shareware assets qualifies as pirating.

Yeah but 'user consent' don't make using assets legal.