The following (hopefully non-controversial) Pipeline build maintenance PRs have remained unmerged for quite some time (in some cases, years):

https://github.com/jenkinsci/pipeline-stage-view-plugin/pull/184
https://github.com/jenkinsci/workflow-basic-steps-plugin/pull/145
https://github.com/jenkinsci/workflow-basic-steps-plugin/pull/156
https://github.com/jenkinsci/workflow-basic-steps-plugin/pull/157
https://github.com/jenkinsci/workflow-basic-steps-plugin/pull/158

done, need release:
https://github.com/jenkinsci/pipeline-model-definition-plugin/pull/467
https://github.com/jenkinsci/pipeline-build-step-plugin/pull/68
https://github.com/jenkinsci/pipeline-milestone-step-plugin/pull/23
https://github.com/jenkinsci/pipeline-milestone-step-plugin/pull/32
https://github.com/jenkinsci/workflow-cps-global-lib-plugin/pull/127
https://github.com/jenkinsci/workflow-scm-step-plugin/pull/56
https://github.com/jenkinsci/workflow-scm-step-plugin/pull/57

done, released:
https://github.com/jenkinsci/pipeline-graph-analysis-plugin/pull/40
https://github.com/jenkinsci/pipeline-input-step-plugin/pull/51
https://github.com/jenkinsci/pipeline-input-step-plugin/pull/52
https://github.com/jenkinsci/pipeline-input-step-plugin/pull/57
https://github.com/jenkinsci/pipeline-input-step-plugin/pull/58
https://github.com/jenkinsci/pipeline-stage-step-plugin/pull/23
https://github.com/jenkinsci/pipeline-stage-step-plugin/pull/25
https://github.com/jenkinsci/pipeline-stage-step-plugin/pull/26
https://github.com/jenkinsci/workflow-cps-global-lib-plugin/pull/124
https://github.com/jenkinsci/workflow-cps-global-lib-plugin/pull/125
https://github.com/jenkinsci/workflow-cps-global-lib-plugin/pull/128
https://github.com/jenkinsci/workflow-cps-plugin/pull/356
https://github.com/jenkinsci/workflow-cps-plugin/pull/451
https://github.com/jenkinsci/workflow-durable-task-step-plugin/pull/182
https://github.com/jenkinsci/workflow-durable-task-step-plugin/pull/183
https://github.com/jenkinsci/workflow-durable-task-step-plugin/pull/187
https://github.com/jenkinsci/workflow-job-plugin/pull/224
https://github.com/jenkinsci/workflow-job-plugin/pull/225
https://github.com/jenkinsci/workflow-step-api-plugin/pull/62
https://github.com/jenkinsci/workflow-step-api-plugin/pull/65
https://github.com/jenkinsci/workflow-step-api-plugin/pull/69
https://github.com/jenkinsci/workflow-step-api-plugin/pull/71
https://github.com/jenkinsci/workflow-multibranch-plugin/pull/128
https://github.com/jenkinsci/workflow-multibranch-plugin/pull/141
https://github.com/jenkinsci/workflow-multibranch-plugin/pull/142
https://github.com/jenkinsci/workflow-multibranch-plugin/pull/144

done, closed:
https://github.com/jenkinsci/pipeline-stage-step-plugin/pull/27
https://github.com/jenkinsci/pipeline-stage-step-plugin/pull/28
https://github.com/jenkinsci/pipeline-graph-analysis-plugin/pull/41
https://github.com/jenkinsci/workflow-step-api-plugin/pull/70