Remember, Comments are ephemeral ...
General Obsolete comments
The most common 'obsolete' comments on CodeReview are comments like:
Hi, and welcome to CodeReview. Your question is off-topic because your code is only available through a link. Please update your question with the actual code.
If you see comments like that, which are still around after the question has been updated, then the comment is obsolete. Flag it.
Other comments are similar, but are on answers where there were problems with an answer, and over time the answer was adjusted to include the recommendations from the comments. Once the answer incorporates the information from the comments, then, technically, the comments are obsolete. Using your discretion, Flag them.
These comments are not obsolete, but rather, are 'too chatty'.
CodeReview is not a forum, and appreciation should be shown with voting.
Personally, I like receiving these comments, but once the message has been received, they are too chatty. Using your discretion, Flag them (too chatty).