Not on Steam, but the longest owned game(s) installed on my Steam Deck are Might and Magic 4+5: World of Xeen. Of the two we had Might and Magic 5: Darkside of Xeen the longest and had gotten Might and Magic 4: Clouds of Xeen from a friend. But a few years later I bought Clouds at a flea market. The version I have installed at the moment comes from GOG.
Back then it was the floppy release. We really had to make space on our 250 MB HDD to install both games at the same time. When you did that they combined into one big game where you could travel between the two sides of one world and solve them in either order. And you had exclusive quests that only opened up when you had both games, including a third true ending to unify both flat sides into a globe.
I considered that our first “real” PC game. Everything before that had only been simple shareware. I do also have Commander Keen 6 installed, but back then we only had the shareware version.