Hey so this just happened to me the other day and honestly I'm really upset by it.
I logged into Mario Maker after not playing for a couple weeks to find that 3 of my stages got taken down. No reason was given except it was server side and it was impossible to reupload them even if I changed the layout or name. There was nothing inappropriate about the titles, nor the content of the levels either. They weren't all new stages or all old, one of them was one of my first uploads and one my most recent upload, which might I add I worked very hard on creating.
Looking around I couldn't find any good answers as to why this happens online. Some people suggested it was Nintendo pulling older less popular stages,but one of them was very new, and also limiting a players upload slots is already a feature of the game so that we can't spam the internet with dozens of stages.
Please if I can get any insight as to why stages are just being pulled and not allowed back online let me know. Mario has always been a major part of my life since I was a small child, and I've always loved the idea of designing my own levels, drawing them out on paper all through out my child hood, so this game was pretty much a dream come true for me. Yet now I don't even want to make any more content because I feel like there's a good chance it'll just get immediately blacklisted.