Trusty has this version.
busybox | 1:1.21.0-1ubuntu1 | trusty | source busybox | 1:1.21.0-1ubuntu1 | trusty/universe | amd64, arm64, armhf, i386, powerpc
Trusty has this version.
busybox | 1:1.21.0-1ubuntu1 | trusty | source
busybox | 1:1.21.0-1ubuntu1 | trusty/universe | amd64, arm64, armhf, i386, powerpc