Well, my first WoW character is listed at WarcraftRealms with a last seen date of December 2005. Another friend from that guild has a last seen date of June 2005:
http://www.warcraftrealms.com/charhistory/2654503
So, clearly there isn't any automatic pruning. I'm glad there isn't, since I want to see the history of the people that I knew in that guild. If my friend with that character wanted to let people know that they had moved to a new server, he could claim the character here, claim it on a new server, and then list the one as "moved" in his profile. That way no information is lost.
As it currently stands, to move a character out of a guild, the owner of the character would need to log on to WoW, drop guild (if he hadn't been removed already), run a census, and upload it to WR. Obviously this isn't practical if his account is terminated, so nearly every guild listed in Warcraftrealms contains names of people that have left.
There isn't any feature to manually move members from "current guild members" to "ex-guild members". If you click on your guild's name, you can add a link to your guild's website, add a short description of the guild, and record the dates your guild progression for level 60 content (Onyxia, Azuregos, Lord Kazzak, Molten Core - Ragnaros, Blackwing Lair - Nefarian, AQ - C'Thun)
If Rollie allowed warcraftrealm users to remove people from a guild, there would be no way to verify it. Someone who wanted to cause trouble could really do damage if that was allowed. If it was important, I would think Rollie would be more inclined to make an armory crawler that would query the guild name (ie
http://www.wowarmory.com/guild-info.xml ... d+Name&p=1 ) and then use that for marking non-members as status unknown, or something of that sort. I don't think there is a lot of benefit to it, though since people who want the current guild roster would just use wowarmory in the first place!