If, like me, you're pretty much done with wow retail, and are tired of Twitch defaulting to show your retail addons, but also don't want to delete it in case you pick back up later, here is how to remove WoW Retail from your Twitch app's Addon section.
These instructions are for Windows 10.
navigate to C:\Users\(Your windows account name)\AppData\Roaming\Twitch\GameInstances
Open the file AddonGameInstance.json (i would recommend using Notepad++ for that), locate the section for your Retail Wow client, highlight the whole section, and delete.
Later, if you play again, just scan for games again in twitch.