Gerrit 3.1.0 was released yesterday (14th November 2019), which means that per the support policy mentioned on the project homepage, Gerrit 2.15 has reached 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 earlier post about the EOL policy.