Is there any sort of API doc out there for 2.1? I've read the release notes, and I'm familiar with the changes to N/sftp and other areas. I'm wondering if those changes are the only changes to the API. (Also aware of new ES6 features)

It actually supports up to ES10, including BigInt. My understanding is that the 2.1 API is essentially equivalent to the 2.0 API just using a different engine.