Actually, the use of Hanza(Hanzi)-Hangul mixed script wasn't exactly a byproduct of the Japanese occupation. (In fact, Japanese was the official language back then lol)

People started moving away from using difficult-to-type Hanza as soon as the typewriter was introduced. As computerization progressed, the transition naturally continued until Hanza was phased out of most documents. Even so, it has only been about 40 years since Hanza disappeared from everyday daily life.