originally in vanilla through burning crusade and possibly wrath. Orb drop rates were somewhere between 2-5 orbs per run with rare chances to only get 1 or 6, average was roughly 3 consistent. You would essentially see 1 orb drop every run due to archivist.
archivist was a 50% chance not 20% for an orb. this was what people wrote but it might have been higher then 50% seeing as he almost always dropped one.
currently the drop rates are average 0-3 with an average of 1. this is post nerf rates.
There was a change was made in later expansions due to the instance being trivial. making the content require more effort to maintain legitimacy in value for time.
note to add: crusader during this change was still usable on twinks and viable end game.