Note that [dead] itself is not necessarily a shadowban: an unpopular comment will show up like that too. To figure out if the user is shadownbanned you want to go to their profile and check if the account is new or has multiple [dead] but reasonable-looking comments.

Comments can be downvoted to death without being flagged. [dead] by itself does not imply its a shadowban. It's super easy to tell though, because if someone is banned, you can look at their history and see all their posts besides a few that get vouched are dead.