Has it really been that long? Apparently so. Valve originally announced their rebranding of Steam Play with Proton back on August 21st, 2018. Seems like a good time for a quick reflection being halfway to a decade old now with the tech that gave rise to the Steam Deck.
Games have to talk to your operating system to have it tell your GPU to draw lots of funny pictures that come together to make up the graphical portions of the game. Game developers do not want to do this directly, because talking directly to the OS is hard. As such, games talk to graphical APIs like Vulkan or DirectX to do the hard bit for them.
For years almost all games used DirectX, which is made by Microsoft. This gave Windows a virtual monopoly on PC gaming because they weren’t about to let their competitors use their API. Then Vulkan came out, which was designed from the beginning to be OS-agnostic, sending us to the promised land of games that could (with some other efforts) run on any machine, anywhere.