The crank bearings, rod bearing, wrist pin bearing and cylinder all need proper lubrication. On bikes that run premix the fuel charge goes through the crankcase on it's way to the intake ports and takes care of that task. Holes are drilled in the centercases accordingly to ensure that the crank bearings come in contact with the fuel mixture and get lubricated. Oil injected bikes typically have oil lines going directly to the crank bearings, that oil is then carried out of the crankcase into the cylinder. When switching from oil injected to premix you have to verify that the crankcases are configured (drilled) to allow the fuel mixture to contact and oil the main bearings. If this isn't done there is a risk of loosing the whole bottom end.
It depends on the manufacturer and how they design their oiling system.
Your mid-70's Yamaha will run fine with the oil pump removed and premix. Their system just injected oil into the intake port, and it was carried into the engine by the airstream. Suzukis from the same era are different. Their CCI system fed oil to the magneto side crank bearing. Removing the pump and running premix means the bearing will be starved for oil.
We used to remove the oil pump and plug the hole with a homemade plate (I think Yamaha made a cover plate for this purpose at the time.) After all, if the pump runs dry it's going to seize up and that's no good.
for what its worth I have also heard some recomendations from back in the day for some models to leave the oil pump mounted but remove the throttle cable end from the oil pump so that it is always at minimum output level and keep oil in the tank...
so that in addition to premix, you are getting a bit out of the pump as well.
Ive plated all mine off. I even run premix in my streetbike.
lower end berrings, are not sealed- they get lubricated from the fuel in the cyl,and cases- your rod as well
some old bikes had the plates that hold the berrings in the cases ,but they still got lub from the fuel too