I have flagged an answer because the comments (9 from who asked the question, and 8 from me) were made obsolete from the edits done to the question, and the consequent edits done to the answer. As suggested on Meta Stack Overflow, I flagged the answer using a custom reason, and explaining exactly why I was flagging the post. The flag has been denied because, "flags should only be used to make moderators aware of content that requires their intervention."
How doesn't the flag make them aware of something that requires their intervention? It is just moderators who can delete the comments left from other users (the purpose of flagging was not deleting my own comments). Flagging a post to remove the obsolete comments is not different from flagging each comment as obsolete. The only difference is that moderators get a single flag instead of (in this case) 9 different flags that need to be checked, and handled singly. As I used a custom reason for flagging the answer, I added an explanation that was clearer than the synthetic "obsolete" the moderators would read in the case I should have flagged each comment.
The comments for this answer can be deleted as they are now obsolete: the question has been changed from the OP.
The suggestion to flag the post instead of the single comments is similar to the suggestion I found on Meta Stack Overflow for flagging two accounts that need to be merged. As it is not possible to flag users, the suggestion is to flag one of the posts done using one of the accounts, and (using a custom reason) explain which accounts need to be merged. The reason of the suggestion is to avoid creating on the meta site a question that is only useful until the accounts are not merged; after that, the question is not anymore useful. A question on meta would make sense if moderators can decide the action to take basing on the votes received from the question, or from the answers given to that answer. In the case of obsolete comments, or accounts to merge there is nothing that needs to be voted.