Mainstream application programmers switched to C in the early 80’s. Game developers were slower to switch, because their small teams and focus on performance kept assembly language viable till the following decade. When id Software released DOOM, they surprised much of the industry by having no reliance on assembly code–despite excellent game performance, and by successfully cross-developing the game (in NeXTstep and DOS), then successfully porting it to an astounding variety of platforms.
Those games didn’t have the splash that Doom did for this sort of thing.
https://web.archive.org/web/20010105180900/http://www.gamespy.com/legacy/articles/devweek_c.shtm
I would say that Wolf3d would certainly count as proof of concept.