As a developer working on a branch, I want to be able to have Sonar check my work without that check effecting the Sonar report on the main project (master branch) so that I know my code quality is high, and when it's not it doesn't mislead developers looking at the main project into thinking the overall project quality has dropped.
The sonar.branch property is deprecated in 6.7: https://docs.sonarqube.org/display/SONAR/Analysis+Parameters documentation seems to suggest to use the (paid) Developer Edition.
There is the Sonar Branch plugin available in the Developer Edition, however this is a fee product: https://www.sonarsource.com/plans-and-pricing/developer/
Figure out how to do this in Sonar (it seems to have changed in 6.7, and might require upgrading to 7+)
using the deprecated feature might be the only path forward if we can't buy the Developer edition.
Make it work