jenkins declarative pipeline has a nice  condition to check for branch names:

stage('build Snapshot') {
    when { not { branch 'master' } }      
    steps {
         sh 'mvn clean install -Dmaven.test.failure.ignore'
    }
}

unfortunate, it only checks for the branch name in `env.BRANCH`. Sure, I can use something like this:

when {
  expression { return env.GIT_BRANCH == "master" }
}

but that's not as nice as the former...

The other solution for this would be, the pipeline-model-definition-plugin would check for multiple variables (BRANCH_NAME, BRANCH, GIT_BRANCH) and maybe throw an error in case they define different values (except for null).

This does not sound like a big issue, but for sure it will make a lot of things easier to understand, e.g. why would the condition behave different in a multibranch job then in a normal job?

please also see JENKINS-50622