Why has the Centeron pulley disappeared on many Shimano rear derailleurs?
For anything over 6/7/8-speed the chain and cassette are now so narrow that any amount of side to side float from the sliding Centeron pulley would not allow indexed shifting to change gears. 

In my experience newer generations of chain and cassette seem to be better at holding in one gear under pedaling load with the derailleur visibly misaligned too, with just a little noise but no slipping, where older and low-end systems would grate and skip teeth. 

As mentioned in a joking-but-not-really-joking comment the bearings in the jockey wheels will become pseudo-Centeron sliding bearings as they wear. When this happens it makes single gear shifts impossible with a single indexed click - you then have to double-click the shifters to change one gear, or triple-click for two gears as the slack is taken up.   (this also happens with other bits of the drivetrain worn)