Oh I'm so sad for them. Mapzen has a bunch of really great products and services, they are some of the top experts in open source mapping. It's a shame the business hasn't worked out.

They have a good companion post on alternative services for folks needing to migrate. It doubles as a survey of the best in open source mapping tools out there right now. https://mapzen.com/blog/migration/

Edit also a post specific to the Who's On First project: https://www.whosonfirst.org/blog/2018/01/02/chapter-two/