I'll drop my Quake II anecdote here. Quake II was the first FPS I ever beat (at least I think I beat it). I did it over a week in the summer when visiting a friend out of state (never would have been allowed otherwise).

When I started, I had only ever played the original DOOM, so I used those controls, and lo and behold they worked, and I thought nothing of it. At some point as I was playing along, my view suddenly became locked to the ceiling and I was thoroughly confused and stuck. Turns out I had bumped the mouse, and managed to figure out that mouse look was a thing after extended fiddling with the system to try to determine the cause and get back to killing badguys. Needless to say, the game got quite a bit easier after that.

There wasn't internet, neither I nor my friend knew what mouse-look was, we had never seen anyone play a true 3d fps. It is hard to imagine how obscure and alien the interfaces for games are because of how pervasive the knowledge has become.

Glad to see the source code released!