It could just be that they started using other proprietary components in their code though.
Would've loved to read the source code for Doom 2016!
I always found amusing that to replace him Zenimax built a whole new studio in Germany to hire the top programmers from Crytek and even then Doom 2016 used a litany of middleware :-P.
It was like the game didn't know if it wanted to be an FPS or some seamless open world ARPG so it tried to do a bit of both and the only reason the FPS side didn't felt bad was because id was masters at making FPSs by that time - but it was certainly dragged down.
It would have been way better if it was a regular FPS with a linear progression of levels like id's previous games.
https://www.gamedeveloper.com/design/patent-issue-forces-new...
With the newer engines there isn't a convenient branch that already has the proprietary parts substituted, and there's a lot more of them.