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.
A lot of the development for Proton has also been community-based. Aside from whatever Steam has done to directly improve Proton, just creating the Steam Deck, and SteamOS has brought so much more attention and focus to improving it to an extent that probably wouldn’t have happened otherwise. It gave people a reason to volunteer their time to improve it.