Tracks played backward.   Slowed down to incomprehensibility--on purpose. Addiction to echo chambers.   All of these may have been done before.   But "Pepper", possibly because of a slight problem with remembering anything about music-- due to chemical enhancement of the "artists"-- put all the above "improvements" together at the same time---on some tracks, not all.

And its roaring success commercially was the starting gun for everybody else to do the same.

Consider what happened in 1967 and succeeding years with "psychedelic music". Anybody here think "psychedelia" was an improvement for music?

And consider what the charts were like in 1966--before "Pepper".   Beatles, Stones, Kinks, Beach Boys, Mamas and Papas, Lovin' Spoonful, Simon and Garfunkel, Four Tops,
Supremes, Temptations, Miracles, Gladys Knight, Marvelettes,( and the list goes on) all had big hits that year. The creative ferment in types of music was stunning--for the last time.