Lisp has had this features since day 1. But Lisp-like langs like Clojure, Racket, etc. don't have it. This is one of the fundamental features of Common Lisp and I don't know why most other Lisp-wanna-be's don't implement it.
I don’t understand the particulars, but one selling point of biff is it’s got built-in support for updating things directly in prod via the REPL.
There’s a fun interview with the biff guy on the podcast “the REPL”. He talks about how much fun it is to develop directly on the prod server, and how horrified people are by it lol.
I manage a few websites written in Lisp, and updating them is as simple as push code, recompile and it works.
Simply loading new code is easy, ensuring the whole system works seems to require a bit more effort.
It turns out, though, that making hot-code reloading work well is mainly a question of how you design your system: designing for hot code reloading isn't all that hard for 90% of cases once you figure out the relevant techniques.
Major releases do go through a full shutdown and release cycle though.