Currently, the latest version available in portage is 0.21.3, however several new releases are available. The latest stable is 0.24.6.
At the moment latest version is 0.27.5. https://github.com/yarnpkg/yarn/releases/tag/v0.27.5
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=f89419b65985cdaff454830e41c8a4e6fa01ba84