Whatever the plaster, you would want something that does not easily absorb water after it dries. Otherwise your wall coating may just weaken and slide off.
A mud with high clay content would work. Horse dung (mixed with a bit of clay?) might work due to its high fiber content.
It seems as though this would be a rather high maintenance structural material needing to be replenished regularly much like a thatched roof.
I’m not sure what they used, but basically they whitewashed the exteriors. The buildings went up fast, were dirt cheap (literally), warm in cold weather, tight as a drum, and easy to fix when needed.. :^) Same goes for thatch roofs, which were still widely used even 200 years ago.