This is the first milestone that includes the improved collections library. Release notes at https://github.com/scala/scala/releases/tag/v2.13.0-M4
Core libraries will appear in the coming days and weeks, possibly a bit more gradually this time because of the substantial change. Available libraries will be added to https://github.com/scala/make-release-notes/blob/2.13.x/projects-2.13.md.