I am not sure the change from classic pet exp was ever mentioned in the patch notes, but you can look
here. There is a
page on Alla's talking about the change. This was originally posted by a dev sometime before June 2002, but the forum post is long gone.
"In the old scheme, if a pet did more than half of the damage to a monster, it took half the experience reward. In the new scheme, pets take zero experience unless no player does damage."
Personally I'd rather the 'new scheme' where 1 damage = full exp, but whatever it's a free server.