source{
          git {
            credentialsId('x')
            remote('ssh://x@x:22/x/x/' + projectName)
            traits {
              gitBranchDiscovery()
              extensions {
                sparseCheckoutPaths {
                  sparseCheckoutPaths{
                    sparseCheckoutPath {
                      path(jenkinsFile)
                    }
                  }
                }
              }    
          
            }
          }
        }

They moved this inside of traits, before was outside traits when leaved outside of traits triggers depreactiaton , can be checkd at:
https://your.jenkins.installation/plugin/job-dsl/api-viewer/index.html#path/jenkins.scm.api.SCMSource$$List.git