Reddit will warn users who repeatedly upvote banned content

I think it's likely:

1. someone posted something that later gets banned
2. in between the posting and the banning, you upvoted it
3. at time of banning/removal or after, they can check the logs of users who upvoted it while it was "live"

Also on reddit, removed posts and comments by subreddit mods can still be viewed if you have a direct link to it, or if you find it from a user's profile instead of from the subreddit it was posted in. From there you may be able to vote on it still, although it's pointless in most instances. However, Admin removed stuff likely cannot be found in the same way. Also a user deleting their own post or comment cannot be viewed in the same way either.

Eventually, they'll ban everyone for not downvoting rule breaking content.