That would've been house bill 1739 that passed during the 2019 regular session.
HB1739 added a definition for "untraceable firearms". It also made it illegal for someone to facilitate, aid, or abet the manufacture or assembly of an undetectable firearm or untraceable firearm by a person who is prohibited from possessing firearms. This is why some companies have decided to not ship kits to WA and why some companies will ask for your DL and CPL before shipping you a kit. There are still companies out there who will ship to WA no questions asked but, after some of the raids targeting 80% stuff, i feel like that may change.