There are 2 repositories under mmorpg-engine topic.
A free, open source 2D online RPG engine in C#/.NET
🌎 runescape classic private server mmorpg emulator
Plugin to transform RPG Maker MZ to MMORPG Maker MZ
Backend support for real-time Action Battle System (MMORPG Maker MZ)
*Game Engine *WIP
DimensioneX Massive Mutiplayer Game Engine (MMORPG)