Bernd Alois Zimmermann
Though perhaps not the original source of the term, the first important discussion of the subject is Alfred Schnittke's essay "Polystylistic Tendencies in Modern Music (1971)". The composers cited by Schnittke as those who make use of polystylism are Alban Berg, Luciano Berio, Pierre Boulez, Edison Denisov, Hans Werner Henze, Mauricio Kagel, Jan Klusák, György Ligeti, Carl Orff, Arvo Pärt, Krzysztof Penderecki, Henri Pousseur, Rodion Shchedrin, Dmitri Shostakovich, Sergei Slonimsky, Karlheinz Stockhausen, Igor Stravinsky, Boris Tishchenko, Anton Webern, and Bernd Alois Zimmermann.