It's an aesthetic that came from the initial idea of romanticising learning and education and visually it heavily leans into the elements of old boarding schools, old established universities and stemmed from books and films like Dead Poets Society, The Secret History, Maurice etc. It tends to be Eurocentric and admire certain subjects (Classics, English Literature etc.)/school types (Etonesque boarding schools, Oxford, Cambridge etc.)/habits (being passionate about your subject, dressing in certain fashions) so people claim it can be exclusive of non-white non-middle class fans.
If people take the aesthetic as a list of rules on how to be academic the correct way, then yes, I guess it can be exclusive as the mainstream dark academica vibe still leans into the 'early 20th Century boarding school/Oxbridge visuals. I just like it as a collection of things/subjects I enjoy so don't care much for the "rules" of it (and not a part of its community online) but I guess young fans can be discouraged by it if they follow dark academia blogs and feel like they don't fit in.