OSM hasn't caught up to it just yet (EDIT: now it has).
You can thank me for that.
I went on and added it after seeing a post on the AARoads forum about it opening.
I'll also need to nail down whether the old route through town has been or will be re-signed as a business route, or whether there is any change to CA 20 through downtown Willits (part of which was concurrent with US 101). So while this is something I'll need to take care of soon, I'll hold off for now.
From looking at the
map I used to determine the route of the Willits Bypass, it appears that the route of CA 20 has not changed. US 101 has obviously moved onto the Willits Bypass. I don't know if old US101 will remain as a business route, though.