When I've flagged a comment, the UI used to show a little red flag symbol, but it doesn't anymore.
This icon is helpful when re-reading a list of comments so that you know that you can skip the flagged ones. For example, say you're trying to fix a problem and reading comments on an answer, but there's a thread that goes off about a mistake in the answer that has since been fixed. Other comments provide useful info, so you're flagging to "separate the wheat from the chaff", so to speak. Then you can re-read the comments to straighten out the info in your head.
Old
(Just to mention, I flagged these because the info has been incorporated into the question, so they're no longer needed.)
New
The only indication is inside the three-dots menu where it says "unflag" instead of "flag":
Outside the menu:
(Just to mention, I flagged this because it's only saying "thanks" and not adding anything useful.)
This is part of a series about regressions in the new comments UI: clickable timestamp, OP highlighting, word-wrap, Enter truncates, deletion confirmation. Also: "Over a year ago"
nmonths before this change and how many flags have we received since?