Nintendo has began taking down items on the Steam Workshop that is related to them, purging items as old as 20 years. Nintendo does have the rights to their own properties however, so they can do as they wish regardless of the history or sentimental value some of these items may carry.
Source - https://store.steampowered.com/news/app/4000/view/4200245595694413052Some of you may have noticed that certain Nintendo related workshop items have recently been taken down. This is not a mistake, the takedowns came from Nintendo.
Honestly, this is fair enough. This is Nintendo's content and what they allow and don't allow is up to them. They don't want you playing with that stuff in Garry's Mod - that's their decision, we have to respect that and take down as much as we can.
This is an ongoing process, as we have 20 years of uploads to go through. If you want to help us by deleting your Nintendo related uploads and never uploading them again, that would help us a lot.