It was in beta for a limited number of users exclusively in the US, but it certainly already existed before he took over. The feature was announced as "Birdwatch" at the beginning of 2021, Elons only contribution was renaming it to "Community Notes" and, if we're generous, getting it out of beta, but that probably would have happened anyway.

https://blog.twitter.com/en_us/topics/product/2021/introduci...