Is there a chance of getting a reddit account banned if you upvote a thread in a subreddit you are not subscribed in?
I think so especially if you upvote posts in random subreddits more than the typical user would.
I think it's a case to case basis. If I were a moderator and I see an account just days or weeks old spam upvoting in other threads, I would be suspicious. So I guess it boils down to how old your account is and past activities. My reddit account is years old with not much activity but I don't get banned despite upvoting/downvoting/commenting in posts under subreddits I am not subscribed in.