I remember a while back that homebrew would not be approved if it linked to other homebrew content, is this still a thing? I can't find anything that says it's still a thing but I am terrible at searching this site?

UPDATE. This was actually really easy to test. I simply added the link, which points to a item within the D&D site, but apparently it can't be published because it references "External Links".

That should really only happen if you actually link to a resource outside of the dndbeyond.com domain, but whatever

I managed to do that with my school of tinkering homebrew. Though I haven't tested it without the homebrew spell since I already have it in my library, but my guess is that they would also have to add the additional homebrew content for it to show.

But you didn't link to the other homebrew item, you simply referenced it, which the new automation system may have not caught. Would you be allowed to link to your specific spell? For example, your's was easy to just reference because there are only three spells when you search for "Animate Trinket" whereas the feat I want to create is going to have many similar named feats. I don't want people to have to search the list for my specific one, I want to link to it.

Fair I miss read what you wanted to know. With my situation it should appear in their character spell list, but I'm unsure if it works automatically or if they need to add the spell to their list too.