There is one from the Rust team below.
As far as I can tell, they are simply claiming it's a project with a code of conduct that seems welcoming. There appear to be projects that don't have a code of conduct that could be described the same way, so it's not useful as evidence of an effect.
My university participated in a study that found their anti-alcohol posters appeared to increase student drinking. Admittedly, with not as large a data set as desired, so it's also possible they simply did nothing. You find this with a huge array of both government and non-profit efforts. Most of the time, the things we try to improve society simply don't work, or not to the degree desired.
I do see a lot of complaints about heated arguments about having a code of conduct, and plenty of evidence of those arguments existing.