Thanks. The Toyota doesn't use a kingpin as such but a birfield joint. What would be a kingpin in other vehicles is replaced with two tapered roller bearings, one top and one bottom. The roller bearing surfaces are free of paint so that the rollers are metal to metal. The (now) painted ball on the fixed axle housing is the fixed part of the seal that keeps the lithium grease inside the ball housing so only a rubber seal and grease are in contact with it.
kingpin and birfield - not the same thing. That's comparing a knuckle mount to a drive axle joint.
Toyota uses a kingpin with trunion bearings. While different than, say, a Dana 60 kingpin, they are both preloaded bearings and must be torqued to spec to get the correct steering drag. The lower kingpin shim is critical for centering the axle shaft in the housing (and not destroying the seal)... the upper shims are primarily for preload. If the top and bottom are coated, you cannot get a proper torque on the bearings, and over time, it can lead to bearing/stud failure.
Bearing surfaces should also be free of coating, as you mention so the races seat property.
Regardless of terms, bottom line... none of the mating surfaces on the knuckle should have any coating on them... the one exception, perhaps, the inner ring that holds the felt in place.