Its shown on several of the videos in the tutorials section but isn't listed in Webflow currently. What happened to it?
Probably because the tutorials accounts use a Typekit integration. Helvetica Neue is a great font, but far from being free.
Helvetica was used before on Webflow, but it was removed from there because it's not showing up on Windows browsers. Helvetica is a system font for OS X and you can buy it for more than $60 I think.
And on Windows, the thin weights of Helvetica Neue can look just turrible.