I was reading on ZDDP (zinc and phosphorous) and found it was introduced by Castrol. ZDDP usage was lowered after 80s as it caused to kill the catalytic converter sooner. On the other hand, modern oils do have other additives to reduce the wear on flat-tappet camshafts such as ester, molybdenum and boron as generic additives and designer additives which are respective to their innovators such as Magnatec or Tehcron etc.etc. using marketing terms on souped up chemical derivatives of these generic additives.
Also it seems ZDDP becomes more important on flat-tappet cam engines when they are in break-in period or newly over-hauled. there is a lot of discussion over the internet on it.