I don't like that ModelChain(system, location, orientation_strategy='flat|south_at_latitude_tilt) modifies the system object. It's not something we do anywhere else in pvlib. orientation_strategy only supports flat and south_at_latitude_tilt, neither of which are commonly used in the real world in 2020.

I think we should remove it, maybe even without deprecation, in 0.8.

I'm ok with keeping the modelchain.get_orientation function for now.