Convertible notes have been around for a longer time whereas safe notes have been invented more recently (in 2013). A convertible note is debt, while a safe note is a convertible security that is not debt. Here are a few articles that explore their differences.
Y Combinator introduced the safe (simple agreement for future equity) in late 2013, and since then, it has been used by almost all YC startups and countless non-YC startups as the main instrument for early-stage fundraising. Read more here.