I'd love to see the research here. Because searching the software engineering Google papers from 2021-2021, the only somewhat relevant papers I found were case studies (such as [1] and [2]), which were based on Googlers' perception of their own processes. While self-perception can indeed by used to measure efficacy, there's a reason why it's used with great caution in surveys. I'll be honest, I have a hard time believing in readability myself.
We have never published the readability research. I could look into doing so, but it is awfully Google specific. Trying to come up with something generally applicable would be a very different project
[1]: https://research.google/pubs/pub47025/
[2]: https://research.google/pubs/pub43835/