Exclusive DLCs from video game pre-orders and kickstarters
 

I'm looking for a place that collects all the exclusive DLCs from video game pre-orders and kickstarters.

Often these contents are completely lost and forgotten, because the possibility of buying them legally is never offered again, but I think they should be preserved to ensure the complete integrity of a video game.

But I don't know sites that store these materials, can anyone recommend them to me if they exist?