I think new people could still use the Intro/Feedback/Shoutout category to introduce themselves and their works, and similarly for sharing achievements. But it's true that then some creavtie promotion ideas could be overseen.
Overall I don't dislike the idea of a Promotion category but it might be counterproductive. Having a new category only for promotion might encourage people to create more promotion posts, so we can end up with one post for each comic/novel, making it even worse than what we have today and kind of forcing people just filter out that category.
An alternative idea could be to have official promotion posts renewed every certain time. So we can have limited number of official "under x subs" which are recycled every week/month and delete all the rest, but still allowing creative promotion posts (with mods judging what is creative enough).
I don't know how the forum is implemented so I'm not sure, but it doesn't really sound like something too difficult to implement. The difficult part would me more on the mods side. Another thing is if they believe that a new category will be actually useful or not.