The reason I ask is because I doubt staff who have left 4-5 years ago will come back to the forums just to post why they banned so and so, nor might they remember any details past their ban reason (which would still be there even if you remove that said staff from source bans as ban reason doesn't disappear as far as I know)
People who mass freekill accidentally wouldn't be server banned nor should be perm banned from team either if it was determinable that it was truly accidental, of course unless they left the server but even then if it was accidental why would they leave the server instantly to receive a server ban in the first place. Regardless, I do agree with bans having more detail is something that staff should strive for as it saves a lot of time for future protests and prevents drawn out "he said she said" scenarios.
Ninja edit: Regardless, I think Chrono is right that as long as it's not an issue to people with sourcebans to navigate through people who used to have powers and who have powers currently, that keeping the people in sourcebans for clarity is a good enough reason to not remove them.