Mainly because the relup functionality is a bit complicated and not always clear, Learn You Some Erlang explains some context [1]. It does depend a bit on the situation, in some cases it's easy to recompile a module in a running system and it might not be much of a problem. But when there are multiple modules being updated, which depend on each other, which possibly require updates to some data structures, it becomes very messy very quickly. Also when modules contain anonymous functions this can cause some issues (badfun errors) because the old version might get lost, but can still have references in the running system.
> hopefully it's not just because of the ease of containerization/docker that is leading to reduced hot loading
It's because it's not super easy to properly architect in, it's hard to test, and it requires supporting running multiple versions of the code concurrently (and the ability to migrate data on the fly).
