Leeches in general: I also usually add more notes when I have a leech or a leech-in-the-making. A common reason for my leeches is that they have multiple failure points. That means I may fail a card twice as much for two different reasons, resulting in a leech.
For flagging cards: I use the following add-on and renamed the 7th-flag to “Purple (Leech)” for clarity. I can then search leech cards or notes with flag:7.
Regarding the suggestion itself: I am not convinced by the argument for why tagging a note is preferable to flagging a card. Flagging a card, unlike tagging a note, loses no information and creates no superfluous ones. Besides, a note is not meant to contain review history, which the leech status reflects. Finally, the Anki software cannot and should not determine if the note itself needs editing or even if siblings of a leech require addressing.
That said, if the Anki team considers this feature more trouble than it is worth, I would be perfectly content with that argument.