Simplified forms have been common in handwriting for centuries. Korean hanja was also simplified in handwriting when it was in use, even though it never got standardized. The "adoption" thus mostly means the standardization of such established glyphs even in print. (This also partly explains why the second reform in the mainland China [1] wasn't received well and retracted at the end.)

[1] https://en.wikipedia.org/wiki/Second_round_of_simplified_Chi...