The first that came to mind are episodes that have been removed from circulation, like the Pokemon episode with Porygon, or the It's Always Sunny Lethal Weapon episodes, or that first DnD episode from Community.

From a quick search, there are a bunch that are stuck in weird legal limbo: The Drew Carey Show, Dogma, Tales from the Crypt.

Someone also mentioned Song of the South being pulled by Disney. Ooh, or the original Star Wars films from before they were edited. There are also probably a ton of not well known older films that would benefit from being preserved.