Currently there is no way to tell if deletion occured during compositionend for Korean IME on macOS
Korean IME provided by macOS does not provide way for web developers to tell if user deleted during composition or not.
Other IME Compositions will show empty string data, when deletion occurs and ends composition.
For example,
Chrome: compositionend shows letter even though user deleted it.
Safari: same, note that it fires proprietary event deleteCompositionText
but it is not w3c web spec. compatible.
Firefox: same
This seems to be macOS's problem because
third party IME like gureum does not have this problem.
And other(Chinese,Japanese for instance) will show empty string per user's action.
VMware