GoKawiilIndie studio's decade-old backend ran seven games on 11 platforms for €10-50/month
A solo developer built a game backend called GDT (Game Development Toolkit) that has run every title they've shipped since 2016, including Marooners, Verdun, Tannenberg, Isonzo, Crash Drive 2 and 3, plus a seventh game built by another studio. The PHP version, roughly 20,000 lines running on Google App Engine with Firestore, supported Steam, Epic, PlayStation, Xbox, Switch, mobile and web without a player-noticed outage in ten years, and has now been replaced by a C# version on Cloud Run.