> Basically when I'm figuring out what language features I care about, I'm thinking about how hard it will be for other engineers to work on the codebase. Things that are not in spec are going to be less familiar and more time to learn.

I agree, this is super important. My inclination whenever I did into a JS project has been to use lodash/underscore everywhere for everything, assuming that it is popular enough that someone will be able to maintain it without much headache, and I can actually get stuff done without breaking my brain over JavaScript's notorious quirks. I'm curious at what point this stops being a good practice. It certainly was 4 years ago.