Released into Early Access on , Project Zomboid has been in development for well over a decade. To put that in perspective: when this game launched on Steam Early Access, Skyrim was brand new, the PS Vita hadn't launched yet, and the word "Battle Royale" meant nothing to the average gamer.
We propose the term (coined here) for Early Access titles that outlast their original design horizon by 3x or more while retaining active playerbases. longest early access game on steam
A close contender is Kenshi (entered EA 2013, exited 2018). Prison Architect (entered 2012, exited 2015) had a shorter EA span. Project Zomboid thus holds the record for longest continuous, non-abandoned Early Access. Released into Early Access on , Project Zomboid