The zinc along with other additives was removed for emissions from a request from the auto makers. The catalytic converters were getting fouled due to the oil formulas, so they asked to remove them since the new engines no longer needed the protection anyway. It wasn't long after that the flat tappet cam makers started seeing problems with flat lobes and worn lifters.
If you have a high HP whumpa-wumpa motor then yes, you might need the added zinc (but at that point you should have roller cams, not flat tappets.. rollers see little benefit from added zinc)... but stock or warmed over motors... no.
The oil we use today has no more or less zinc than the original formula's from the 1940's and 1950's. The use of zinc spiked in the 70's as it was a cheap way to prevent the oil from oxidizing. zinc was never added to reduce wear, just to stop the oil from breaking down. To much zinc will eat the metal (via acid etching).
You are correct that the zinc exhaust gases might foul the O2 sensor or catalytic converter... but so can running rich (soot) excessive oil consumption and a host of other things.
Cam manufacturers blame the zinc for a period of time when cams were not lasting... I don't buy it. There never was a huge issue of cams being wiped that can be directly blamed on API certified oil.
