It would be a fallacy to imply that feedback is a sound technique solely pioneered by rock guitarists. There are multiple influential uses of feedback manipulation that appear throughout 20th Century classical music.
US composer Robert Ashley was possibly first to experiment with feedback as a compositional tool. His 1964 work The Wolfman, a piece based around vocal mic feedback, gained a reputation for being potentially threatening to the listener’s health and hearing.
American minimalist Steve Reich’s Pendulum Music is, however, probably the most notable modern classical work to take feedback as its primary sound source. The piece is based around a series of microphones suspended by their leads over speakers, which are rocked like pendulums to create throbbing and slowly evolving feedback that eventually morphs into a constant drone as the mics come to rest at a standstill.