There have been a couple that have been let go over the years, but most mods are removed just due to inactivity. Quite a few mods seem to get burned out and just disappear. I can't blame them. Hopefully they're sat on a beach somewhere or just enjoying their life offline.
Enjoying their life?
Just curios, did theymos pay all the mods for their work or were they doing it as a voluntary job without any demand of money? If they were not getting paid, I think this could also be the reason why some of them left and got fired from their position.
The criteria of pay includes other factors too but as a common member I am not aware of what are the factors included in mods pay and if it only depends on their numbers of reports handled surely they will be handling more reports then only more pay or all of the forum mods are too rich.

just sarcasm no offence fellas.
@OP.mods of mining board and politics were removed after some controversies and Lauda the remaining members were removed only after their long inactive and still some of them being a staff even though they didn't use the forum for years.