The 'traditional' school holds that, since much of the way a piece was intended to be performed by its composer is not found in manuscripts, important aspects of performance came from direct instruction by the composer; Bach would have explained to players how he intended the music to be played, and trained them himself, or through assistants trained by him.
This unwritten knowledge was thenceforth kept alive by repeated performances: players and orchestras and directors learned from each other, and from attending other performances, how the piece was to be played. And so this knowledge was…