I think it's fair for an author to say they aren't interested in concrit especially things that aren't easily fixed after posting like pacing (rather than fixing a typo or such).

I'm not especially into random internet audience offering me drawing tips I didn't ask for (unlike when posting wip trying to figure out a problem and asking), especially not when I might feel a bit down on myself already.

And the feedback etiquette for the "default" modus of engaging is quite different in different fandom corners. So it's helpful to be upfront. Otoh when an author is too particular or seems very easily offended they risk people just not saying anything beyond a kudo or a generic "I liked this" because just posting thoughts you had might seem risky, even if you don't mean it as something they ought to fix.

I think it's a bit of a shame that fandom currently has few reader centric spaces, that don't center around the author or are addressed to them. I enjoyed the mailing lists way back that were for fanfic talk among readers.