However there seems to be two types of "Closed". I'd also search for just "closed" and in the results there are proposals with a grey box and the word "Closed" which aren't in the "closed:1" results. There's eleven of those. So over time it appears there's been 70 or so proposals suggested that never made it to beta launch.
There may be more that age, eventually fall off the front end and are "deleted" and possibly not visible to low rep users, much in the same way the rest of the sites work, i.e. you need at least 10k to be able to see deleted posts [0]. I've only got 1350 rep in Area51 so there may be very old deleted proposals I can't see.
[0]: http://stackoverflow.com/help/privileges/moderator-tools