Not useless but waaaaaaay more clumsy / inconvenient 
You may use the Sonarqube web api like that
As the former way to do that via property sonar.branch.target has vanished with Sonarqube 8 it’s only possible to set this with the web api or via web ui, the default new code setting for a branch is previous version
Otherwise when using PR analysis which is activated with properties
sonar.pullrequest.key, sonar.pullrequest.branch, sonar.pullrequest.base
the new code is defined by branch and base.
see Pull Request Analysis | SonarQube Docs
Still hoping sonar.branch.target comes back.
In the meantime my feature request has already 13 votes.
So i guess there’s a serious requirement of the community as this topic comes up again and again.