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.
I think a lot of people are misunderstanding what Proton actually brought to Linux gaming.
I had been running Linux exclusively for some moths in 2013-2014, and trying to get games to work on Linux felt like this : Wine is likely able to run it if you can found the right configuration, but good luck with that. I think the only game I managed to run without issues was Civilization 4, so I rolled back on Windows some time later.
Of course, Valve contributed to Wine, and projects like dxvk and others are major achievements (if a team effort), but that's not their main contribution. Valve understood that gamers may be somewhat more tech-litterate than other people, but that making games work on Linux should be easy. And that's what Proton was made for.
Nowadays, most games I buy on Steam work out of the box. I sometimes forget to check protondb before buying a games, and I rarely had an issue. Even if in 2018 you had to tinker a bit, you rarely needed more than to choose the correct Proton version (big up to Glorious Eggroll).
I think it's symptomatic of the situation of the Linux Desktop : technically, it's where it needs to be. But there is still a gap in accessibility and easiness. Tinkering is nice, but you should not have to do it to have something that works.
Not only that but they helped reach critical mass to drive adoption, and used their dominance in the industry to push studios to work with us, instead of against us.
To add to that, back in the day you had to find out what engine a particular game used as there were huge compatibility issues with certain engines and others ran a fair amount slower via Wine. Some engines, however, ran incredibly well under Wine.
That said, there were some cool things you could do in Wine like define a pseudo monitor to run your game on. Example, back in 2010 (before widescreen monitors were more common) I had a triple head setup on Linux. I could specify in Wine an arbitrary monitor size (like say 2560x1024) and run games "full-screen" centered on my setup while having other windows open on the edges of my real desktop.
Even games that officially didn't support multiple monitors and on Windows (would force themselves to one screen and black out the other ones) ran well via Wine with this setup.
It was a bit involved to get working the first time though!
Played through the HL2 games, Supreme Commander, Rift: Planes of Telara, and even Wow that way (though WoW had other issues with non 4:3 displays).
Nice to see some comments like yours truly get it. The amount of people I've seen that now use Linux just because of Proton is surprising. It also gave us the Steam Deck. Valve do tons and people saying it's just "Wine patches" or moaning I didn't focus on Wine are missing the big picture that you get nicely :)