We did have our code mostly sorted into 3p and 1p... However I'm not a lawyer and not sure how the taint might leak up. I forget where Google/oracle landed on apis being copyright but havock ids and other concepts leak into your code everywhere. Heck even proprietary details might or algos. It would still be hard and the lawyers would be skeptical. You'd need a business case besides goodwill to get over those humps.

I bet a ton of companies don't follow those best practices. And there's also the fact that you've now exposed yourself to litigation where some dev violated copyright of previous companies or libraries or whatever and put it in the wrong place or copied it. Before open sourcing you were fine. After you have essentially incriminated yourself.

The doom/quake case is kinda an exception. In that you likely (I'm guessing) had Carmack who is a founder/owner, deeply understand the tech base, understands cost and estimates, likely understands licensing quite well, able to go do the work, but also able to make the cost benefit tradeoff and override objections in some cases. That's not common in many companies.