The GIT branch 5.1 has just been created, it will host the release series 5.1.x. To get this branch from GIT, you can use:
git clone https://github.com/kamailio/kamailio.git kamailio cd kamailio git checkout -b 5.1 origin/5.1
Hopefully in two-three weeks time frame the full release of Kamailio v5.1.0 will be out.
From now on, any corresponding fix has to be pushed first to master branch and then cherry-picked to branch 5.1. No new features can get in branch 5.1. Enhancements to documentation or helping tools, as well as kemi exports are still allowed. If you are not sure about doing or not a backport, ask on sr-dev mailing list.
The master branch can now get new features, which will be part of the future 5.2.x release series.
Thanks for flying Kamailio!