Some 15 years ago a German homebuilder developed a motorglider prop with automatic angle adjustment. It was a design where the axis of blade rotation was somehow angled with respect to the prop shaft and with respect to the blade axis, so centrifugal forces were turning the blades to high pitch while thrust was driving them to low pitch, thus giving a self adjusting feature. He extensively tested it (together with the rotary engine griving it) on a test bench and in windtunnels, obviously getting very good results.
Never heard about him to actually finish the plane.
And of course there are still autonomous fully automatic variable pitch propeller available on the market, testet in service for millions of flight hours, for example the
V-503Afrom AVIA in Chech Republic.