I said this over on the skysite but realized it would probably be useful over here too, so!
Friendly reminder to AO3 readers that you absolutely should not be in the habit of making personal Collections for your favorite fics—instead, add tags to your bookmarks and use those to browse! This also works for private bookmarks, so there's no reason to do it otherwise.
The issue with using Collections for bookmark organization is that this feature can be (and often is) used by bad actors to hide fics they dislike by setting the collection to "Unrevealed" after the author agrees to have their work added. This hides the fic from everyone except the collection admin and the author, changing the fic listing to a note reading "This work is part of an unrevealed collection."
As you can probably guess, this is a common fancop tactic to hide what they consider "objectionable content" from readers.
Authors: DO NOT allow people you don't know to add your work to collections unless you actually participated in an event for which the collection was made. If you agree to have your work added to a collection and the admin changes the status of that collection in any way, you do not receive any notice. It doesn't matter how old the fic is, how many people had it bookmarked, how many people were subscribed to the fic—if the collection is set to "Unrevealed," even after the fact, nobody else can see anything in it.
Readers: DO put tags on your bookmarks and use AO3's browsing filters on said bookmarks to find specific fics. There is no reason to make a personal collection with how robust AO3's tag system is, and the only reason you'd set a personal collection to "Unrevealed" is to intentionally hide someone else's work, which is an undeniably shitty thing to do.