Astana said: Isn't it a good idea to add a link to a post containing the list of artists who opted for their works not to be posted in Danbooru? It'll save users from some confusion if their post disappears for some reason other than 3-day-queue.

Well, this is the useful threads index, and that's part of the wiki help documentation. There are a host of important things in the documentation that that we don't link here.

The list is located in howto:upload.