Top: Nintendo has been proven to use the fan website The Spriters Resource to access sprites from their own games, instead of taking the sprites from their own archives. A sprite in Super Mario Maker (later even reused in Super Mario Maker 2) contains a pixel underneath Mario's mustache with a color that does not actually appear in Super Mario World. However, that exact color appeared in a sheet submitted to that site that was up in 2013-2015, at the time of the game's development.
Since the only way we know Nintendo used fanmade rips of their content was because they forgot to remove the pixel in this instance, it stands to reason that many more instances exist that are unknowable due to Nintendo successfully scrubbing out any traces of the original sheets.
Bottom: for everyone who is interested in sprites or is already a fan or user of The Spriters Resource, you may find it interesting to tune in to a livestream that is currently occurring whereby a new design for the site is being showcased. All feedback from the viewers is appreciated and will be considered for changes and future updates!
fucking sic