In my profile settings, I have showdead: yes. It makes [dead] comments show up and I guess [flagged] too. (But I think it only applies to comments of other people, not your own comments)

Keep in mind that you need karma to do many functions in this site. For example, with enough karma you can flag other people's comments, and with enough flags the comments become hidden. With more karma yet, you can vouch for comments so you can make removed comments reappear on the site

Actually I just vouched for this comment https://news.ycombinator.com/item?id=49790164 because I think it's fine. It was previously [dead] but not anymore

(note, it's not usual to tell you vouched or flagged a comment, I'm commenting because it's what this tangent is about)