Milestone 15 was completed on 2020-12-23. It adds support for the ROS distro releases current at the beginning of November 2020, for webOS OSE v2.8.0, and for OpenEmbedded hardknott, and drops/reduces support for OpenEmbedded thud, warrior, and zeus.
A new revision 22 of the OpenEmbedded Build Instructions updated for this has been published. Its change log is here. A “rich diff” of the changes made since the previous revision is here.