Claim: The Japanese attempted to stamp out the Korean language and Hangul by imposing Hanja-Hangul Mixed Script (國漢文混用, 국한문혼용) and the Japanese language.

Rebuttal: Throughout the colonial period, the colonial administration did not completely suppress Hangul and the Korean Language, and in fact promoted them as a means to disseminate Japanese propaganda.

Contrary to popular belief, Korean language education was not suppressed at all times during the Japanese occupation (1910-1945). (I do not mean to diminish the atrocities of the Japanese colonial administration.) The Japanese colonial administration recognized illiteracy as a problem in Korea, and saw Hangul as a useful means to eliminate illiteracy and to disseminate pro-Japanese propaganda. Indeed, Korean language education, along with Japanese, was a mandatory subject in the colonial education system until 1938. Textbooks published by the colonial administration were in Hangul exclusively or in mixed script, often with Japanese translations.
Many of the rules in modern Korean orthography also dates back from this era. There were no efforts to standardize orthography until the Japanese colonial period. The colonial administration promulgated two standardized orthographic rules: the 1912 Primary School Use Korean Orthographic Rules (普通學校用諺文綴字法, 보통학교용언문철자법) and the 1930 Korean Orthographic Rules (諺文綴字法, 언문철자법). These two standards heavily influenced the 1933 Orthographic Rules (한글 맞춤법 통일안) put forth by the Chosun Language Society (朝鮮語學會, 조선어학회), the predecessor to the Hangul Society (–學會, 한글학회). The 1933 Standard became the basis for standardized orthographic rules for North and South Korea.
Notable changes from the prior conventional spelling in the 1912 orthographic rules include: the abolition of the arae a (아래 아, ㆍ), which was originally pronounced like the “aw” in “awe” and had disappeared from the Korean language by the late 19th century; and changes to spelling of palatalized syllables (i.e., 댜 → 자, 쟈 → 자, 샤 → 사, 탸 → 차), whose pronunciations had also changed by the 19th century, except for Sino-Korean words.
The 1930 orthographic rules introduced the names for the Hangul consonant letters (i.e., giyeok (기역), nieun (니은), digeut (디귿), and et cetera) that Koreans still use today. The rule also removed the exception for Sino-Korean words for spelling of palatalized syllables, making the spelling of palatalized syllables uniform. For example, the word soyo (逍遙), meaning “to stroll around,” changed from 쇼요 to 소요.
Furthermore, the rule of pronouncing Japanese proper nouns by their Japanese pronunciation dates from the occupation. For instance, prior to the occupation, it was standard for Koreans to call Osaka “Daepan” (大阪, 대판), the Korean pronunciation of the Hanja for that word. Another famous example is when Ahn Junggeun assassinated “Ito Hirobumi” (伊藤 博文, 1841-1909), Koreans at the time heard that “Ideung Bakmun” (이등박문), the Korean pronunciation, had been assassinated, not “Ito Hirobumi” (이토 히로부미). Not surprisingly, there are a few mixed script proponents that advocate for a return to Korean pronunciations of Japanese proper nouns.
There was one significant difference between the orthographic rules promulgated by the Japanese colonial administration and the one used today. The Korean orthographic rules promulgated by the Japanese were phonological, not morphological. For instance, under the 1912 and 1930 Orthographic Rules “to be good” or “to like” was spelled and pronounced 조타, jotta, the phonological spelling. Under 1933 Orthographic Rules, it is spelled 좋다, which is spelled johda but pronounced jotta. Ironically, even as late as the 1960s, many Hangul exclusivity proponents, who used anti-Japanese sentiment to argue against mixed script, advocated for a return to the phonological spelling rules, which they thought was simpler and was incidentally promulgated by the Japanese colonial administration.