[Liquid_Fire]

> is it common for a mod to expose custom commands in chat or something?

Yes, it's quite common for mods to expose custom commands, although these are not raw Lua functions but rather "slash commands", e.g. /foo

Other than for messaging/chat, the chat box can also be used for various commands for actions your character should perform, e.g. /wave, /target, /sleep and the game also has a built-in macro system that lets you combine several of these into a "macro" that you can then execute by pressing a single button. But these are all "safe", i.e. they don't eval raw Lua code.

There are some WoW quest-tracking websites that will sometimes give you little snippets of Lua to execute via /run that will e.g. query the game about whether you completed the given quest, but in general normal users shouldn't need to use /run, it's more of a feature for mod developers.