I would think that the 15.0 version of newinstall.sh, or a branch started from that tag, could be used.
Presently, the only way to stop attempting to build py2 nightly/weekly binaries is to remove them from the list of tarball targets. If they are not removed, the nightly/weeklies will start failing as soon as py3.6+ is required on master. I would like to continue the work towards making the release workflow configuration file driven by adding some sort of profile concept (eg., master, v15, etc.) but this keeps getting delayed due to high priority requests.