My boat with the Chevy engine had brass freeze plugs. It was to reduce galvanic action. Corrosion. Brass plugs are less likely to corrode if the coolant mixture becomes weak. When I bought the replacement block, it was intended for a Chevy truck and had steel plugs. I removed them and installed brass plugs. Of course, the inboard engine used freshwater or seawater for cooling so there was no antifreeze in the system.
If they tarnish, they are totally out of view, as you noted that they are very difficult to change with the engine installed. Especially the two behind the flywheel.
I'm not sure that OEM plugs are made of stainless. Some aftermarket freeze plugs may be but not OEM.