There is a pilot project for Bike trails & additional foot trails in Cheasty Greenspace. Some trails seem to be open already, but not all.

It could be worth contacting Parks & Rec or one of the other agencies for official info.

Project page: https://www.seattle.gov/parks/about-us/projects/cheasty-mountain-bikepedestrian-trail-pilot-project
Cheasty website: https://www.cheasty.org/