Does your research and a/b testing cover users that access the site via a screen reader?
To be honest, I didn't even think about it until now, but since you mentioned it, I'll ask my design team to do a small-ish research and in the future try to build navigation better accessible for people with impaired vision. I'll hardly be a top-notch experience, but at least we'll try to pick the lowest hanging fruit.
Not saying that hamburger menus can or cannot be accessible; just saying that, as an engineer, you really have an obligation to make something that is accessible. This is a basic, non-negotiable requirement for all software.
Info for the underinformed: https://www.shrm.org/resourcesandtools/hr-topics/behavioral-...
https://www.whoisaccessible.com/guidelines/largest-web-acces...