During the last ESC meeting it was decided that we will aim to release Gerrit 3.1 during the upcoming hackathon in Sunnyvale in November 2019.
The tentative plan is to cut the stable-3.1 branch a few weeks before the hackathon, i.e. around mid-October, so that there is enough time to make release candidates and stabilize it before the release.
Per the support policy mentioned on the project homepage, after 3.1 is released 2.15 will reach end of life and will no longer be actively supported. Support for 2.16 and 3.0 will continue; users of 2.15 or earlier are recommended to upgrade to one of these versions.
See also the recent post about the EOL policy.