It is documented internally. We recently went over the list and the vast majority of diamonds are removed at the request of the moderator or because of inactivity. We encourage those moderators to post something on meta if they want, but there's no reason for the community to pry into the ex-moderators' personal lives. It's a volunteer position and people have lives outside the site that often interfere with their ability to moderate.
A very small number of moderators have been removed involuntarily. The most common reason was violating the moderator agreement, but you can find the details of the other situations on the relevant per-site meta.
We do retain the results of all moderator elections. (Stack Overflow is the only exception. Election #1 was the first to use the current election system, not the first election.) We also track pro tempore moderators on meta unless we forget. So it is usually possible to piece together a history of site moderation from public information. But I can't see why that would be needed to participate on the site day-to-day.