it depends on how you do it. phrasing it like a demand is rude but saying that youd love if more writers tagged a specific dynamic so that you can find specific things to read isnt rude.. and if you enjoyed a specific fic but notice the tag missing, its kind to first compliment the fic and then ask if they can tag the position so that you and other people can find it better. and obv the authors can say no for whatever reason. the tone and context and intent really matter in this case