According to other sources (ProtonDB), about 80% of a much larger subset of Steam games run on Linux (some worse than others) - next milestone is 90% I guess, a couple of years down the road.
Some of those games 'require' an unofficial version of Steam's Proton which includes support for features in the underlying components that the official version can't ship.
Mostly it's an issue for video codecs in games (patents are the issue here), or games that use a less popular API. (From windows! Which has ~3 decades of APIs from multiple vendors. Let alone 10s (10-100 IDK) different ways of playing back audio or a video, or both at the same time.)
Some of the changes are simply more bleeding edge patches or game specific tweaks that are included faster than official releases include them.
