I wonder about the term "lost", as in how much truly is lost. I often think that many "lost" things are just in vaults and they use the "lost" term as a marketing tool. I read that Jackie Gleason intentionally stored many Honeymooner's sketches in a vault for a few decades with the intention of someday making them available, and then upon releasing them said he'd "found" several "lost" episodes.
And sometimes there are specials and stuff that feature "lost" stuff, but obviously they had been found if they are included. I know there was an "I Love Lucy: Lost Episodes" VHS release that was really just guest appearances by Lucille Ball (I wonder if the rarely-seen Christmas episode was included). And I recall reading once that Lucille Ball kept kinescopes of everything she appeared in, so nothing with her was really lost.
And there's the "Toonheads: The Lost Warner Bros. Cartoons" special, which was obviously able to locate many lost shorts. I guess some of them could have been lost at some point. After all, I read that all of the Hook shorts were found in somebody's garage. And I know of one Warner short that was lost, I can't remember if it's "The Door" or "90 Days Wondering", but I read that Jerry Beck wanted to include it on one of the Looney Tunes: Golden Collection releases but couldn't find a copy in the Warner vaults, but that short did end up released as a bonus feature on the first Looney Tunes: Platinum Collection Blu-Ray.
And then there's the Disney-produced Oswald shorts that are lost (if they still exist). I know that a few were located after the Walt Disney Treasures release of Oswald shorts. And i wouldn't be surprised if there are some post-Disney Oswald shorts still lost.
But it seems like the majority of things noted on this wiki are stuff that's not truly lost, just stuff that we can't easily see online or by doing tape trades. A lot of the stuff covered likely exists in vaults/archives and the companies just haven't released them (whether they choose not to or not).