I think it not only varies by the words, but also the movie genre/director.

Every Christopher Nolan movie would need every word in boldface. Can you even hear (or understand) what is happening in Tenet?

>I think Netflix should enhance closed captions by using boldface on words that are more likely to be misheard due to high-frequency hearing loss.

closed captions are used under the assumption that none of the words are being heard. And it's not as if the sound being harder to hear makes the text harder to read, so I don't see the benefit of bold text.