Only illustrations that are legendary and famous enough to mark a history in pixiv can be attached with this tag. The criteria for attaching this tag is rather tricky: it is not just based on how pretty the illustrations are, but rather how much it has impacted the history of pixiv since the start of its establishment. In other words, the illustrations has to be impactful enough to leave a mark in pixiv.
While this tag is not meant to sound official nor it was made by the pixiv staff, this tag has since been used at a certain point of time starting from the first ever submitted illustration in pixiv, and then to illustrations that made a mark on its own. It has since been used on illustrations that received tremendous reactions from the viewers, or even illustrations that are used in printed medias and so on.
One thing to note about this tag however, is that you do not attach this tag onto your own illustrations, the viewers will do it for you instead. Since the purpose and timing of using this tag is highly subjective, cases where this tag got constantly removed in favor for a better tag and reattach aren't exactly uncommon. However, since this tag is never meant to be endorsed nor it has to be attached to ensure this piece of artwork stays legendary; and many viewers, including the original artist themselves have different opinions on different tags, the proper assessment to this tag can be very difficult.
pixiv's Oldest Drawing pixiv Legendary History Everything Started Here Beginning of a Legend Eminent Drawing Skills 10 Points Are Not Enough 50000 Users Bookmark List of Evaluation Tags
pixiv New Year's First To Arrive - While this is not Hall of Fame, the subject is however similar.
Related Articles of Different Usage
Hall of Fame - For Pokemon use tag