When I try to save my household to my library it tells me that they are using custom content. This is definitely not the case because I have never downloaded any custom content for the Sims 4, and it does not say that the individual sims include custom content.
When I found this out by allowing myself to see entries that "include custom content" in my library I saw that previous households I have saved had this issue as well.
There are other simmers that are having the same issue, of the game saying that their sims have cc on them, although they have never downloaded cc for their game.
A workaround if you would like to upload your sims to the gallery and want them to be cc free is to start a new game, go to CAS, replace the random sim that is there with your modded cc Sim from your library , and then save the sim to your library again.
My Gallery Page https://tinyurl.com/Roseymow
My Showcase thread https://forums.thesims.com/en_US/discussion/948861/rosemow-s-rooms-showcase