LPMUDs ran almost entirely on hot reloadable code written in a quirky language called LPC, which later inspired the Pike language.

I believe that only the "driver" code, which handles system calls and hosts the LPC interpreter and is written in C, couldn't be hot reloaded; everything else running in the game could be reloaded without restarting the server.

I'd guess in the modern day, there would be some games where Lua scripts can be hot-reloaded like any other data, from a database or object store.