Zddp has worked very well for flat tappet valve actuation systems in automotive, marine, aircraft, motorcycle and industrial engines for decades. With the implementation of catalytic convertors however there came a conflict. The phosphates and zinc were poison for the convertors. As a result, the automakers moved to roller type tappets which also eliminated friction snd slightly improved fuel economy.

Now, model engines. The two stroke poppers have no cam/lifter interface with the extreme pressure l, sliding contact and benefit little from that quality as imparted from zddp.
Four strokes are another story. They do indeed have that condition and can well benefit from the extreme pressure qualities. Zddp is not compatible for mixing in the fuel

Castor oil has similar properties and is indeed compatible. It only takes a small percentage in the oil package to get the needed extreme pressure quality.

Zddp is still used in oils run in flat tappet engines. Without the Zddp or similar additive, excessive cam /lifter wear can be a problem.

Zddp has long served and works well where needed.