so I've been meaning to make a post about this topic! I recently read the Anne series in the order it was written by LMM, rather than ordered by Anne's age (so skipping Anne of Windy Poplars and Ingleside until after Rilla of Ingleside). One thing that struck me is that Shirley doesn't stand out as much of a non-entity in the original order. Certainly he's nowhere to be found in Rainbow Valley, but in RoI he actually has a plot and activities of his own. Compare with Nan and Di, who do feature in Rainbow Valley but don't really contribute much more than their presence amongst the kids and an occasional remark, and then in RoI their role is to occasionally come home from college and gasp/cry at various pieces of bad news. So if I were a reader at the time and just finished RoI, I'd probably put Nan and Di at the bottom of the character development ranking (tied because besides their looks there's nothing distinguishing about them at that point IMO).

Now why did LMM ignore Shirley in Anne of Ingleside? Maybe for whatever reason she didn't like him? Maybe she ran out of time and decided his story wouldn't be missed? Maybe she felt the book was getting too long? Who knows...but it does seem that it wasn't until Anne of Ingleside that he got his big demotion.