This is one of these questions that comes up every few years.
Back when I started editing the db, we actually did!try removing unavailable games from the db, accept two things happened.
The first, was games started making a come back (I had to completely rewrite the chicken killer page). Both because Pc games would get hosted elsewhere (something even more likely now with the audiogames archive), or with online games like alien adoption agency starting up again.
The second, was a bunch of people were saying "hay what was such and such a game?" and it always coming up in discussions.
It was for this reason the unknown license type was created. If a game is listed as "unknown" it basically means either the game has vanished, or the game has been shutdown and we're not sure what has happened to it.
All games with an unknown license type also have notes at the top of their description saying that the games are not around, so people know when they're looking at the description what the game is (and if any don't have notes let me know and I'll fix them).
Now the one thing I do wish, is that we could have a checkbox to exclude games with the unknown license type showing up in the dropdown, or when you use the "search games" function to display games of a given license type, genre or developer.
Failing that however, we do try to make things as clear as possible so people don't get too disappointed, including as Ironcross said, removing site and download links when the websites are completely down.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)