Hi,
That branch wasn't announced publicly, but somehow you managed to saw that
As described in the README, master branch will track latest development for MATE on top of -current.
Upstream development release is 1.19, but not all tarballs are released, so i put the changes on 1.19-dev. Once all 1.19 tarballs are out, it will be merged into master and development continues there until 1.20 comes up later this year and we make a new branch for that release.