Graham Pearce wrote:

I've been doing a lot of work with history merges in the last few months. I started on place names, but then branched out to planets and other fields. However I've found some pages whose page history has been lost. I have collected some info about these pages, as well as other page history oddities, at: http://en.wikipedia.org/wiki/User:Graham87/Page_history_observations

We need more studies like this. Specially if they can be done in a semi-automatic way.

It would be good if the old history of these pages, especially important pages like Canberra and Glasgow, could be retrieved from the deletion archives.

It's not that they're easy to retrieve, like normal deleted edits or oversight ones. They're lost on the hazards of system updates, some sysadmin would need to dig over old backups, with no certainity of finding them. Entries available at nostalgia are the easiest ones. Next is probably reattributing 'Conversion Script' edits ([[User:Ryguasu/conversion_script_AI]])

Will the deletion archives ever be cleared again, and if so, under what conditions?

My answer is no. Not only it is undesired and unneeded (there is enough free space for the time being, and would be better ways of freeing if the need arised), it may be very hard/almost impossible to do, as the real text will be interleaved with valid revisions in the chunks (you'd only remove the references to it).

It'd take something really big for that (Wikipedia being buyed by another company, a meteorite destroying the cluster with all the backups...).

For the definitive answer, ask Brion :)

Also, less seriously, I am curious about the page history of: [http://en.wikipedia.org/wiki/List_of_numbers_that_are_always_odd as listed on Wikipedia's lamest edit wars. It was deleted for a good reason, but the history would be fun to read.

I don't even know the reason. Maybe it's time to restore [[Special:PrefixIndex/Wikipedia:Deletion log archive]], which in 2006 Ral315 "temporarily deleted temporarily" and would restore "soon".