I’ve heard of people getting banned in Overwatch playing on Linux years ago, so I’m a bit worried about playing WoW on Steam Deck (runs on SteamOS, which is a version of Linux) because I dunno if there is the risk that I could get banned out of the blue sometime in the future.
Is there any sort of official stance on if it’s perfectly ok to play WoW on Steam Deck?
yes you wil get banned, cloud gaming for wow is unfortunately a nono 
You are probably confusing Steam Deck with Steam Link. Steam Deck is Valve’s handheld console type device running Linux and running games locally, while Steam Link is used for streaming games over local network (“cloud gaming”).
Unlikely, but there are several replies by CS people both in EU and US that just playing on Linux is not bannable, and just that will not get you banned. There has been rare cases years ago when, if I remember right, WoW and Diablo 3 players using Linux were banned mistankely but they were overturned. I have seen several reports of players being able to play WoW succesfully on Steam Deck in eg. WoW’s subreddit.
Obviously playing WoW using Wine/Proton etc. is not supported so on any technical issues or such you are on your own.
Alternatively if you don’t want to configure Wine/Proton you can install Windows to your Steam Deck.
Some players are already playing it and having a great experience on the Steam Deck:
aaah yes you are right, my bad
It’s running locally on the hardware and not over the cloud. It goes through the battle net launcher and you can even run it through windows if you want so it’s no different to a PC or laptop. Complies with my understanding of the TOS.