It's exceedingly rare, but there's one case where a game can disappear from your Steam library: keys.
Game developers/publishers have the ability to revoke keys at any point, and that can result in games disappearing from your library.
I'm yet to hear of Valve themselves making a purchase inaccessible, but I have had keys get revoked. For example, free Gleam keys that add shovelware to your library.
Even games that Valve banned from sale on Steam can be activated and added to your account using keys. I believe that people who purchased the game also get to keep their purchase even if it's later banned from sale.
This is literally the first post I saw when opening the app. I guess I'll do something else.