The § symbol is not appearing on any text window I try typing it into (signs, book and quills, command blocks, etc). However when I check the debug menu (provided by the launcher options) when I am on a server (Mineplex for example) the § symbol appears when colored text is displayed from the server on the main screen (for example, if text appeared on the main screen that said ABCDEFG (bold scarlet text), it would appear in the debug menu as §l§cABCDEFG). I know this works in normal play because I've seen it done on other computers (Macs & PC's), but I can't even type in the § symbol to begin with. I can type in the symbol in other applications, and I know I'm using the right keys (Option/Alt +6). What is happening?
§-based formatting is deprecated. You're supposed to use chat components now. You aren't able to enter the symbol in-game because its use is deprecated and chat is not designed to accept it; this is deliberate and working as intended. You might be able to use it in written books, which do still use it, though.
Some servers support using
&
to trigger this formatting. This is not vanilla behavior, though.