on:
  issues: {types: milestoned}

jobs:
  apply-label:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/github-script@0.2.0
        with:
          github-token: ${{github.token}}
          script: |
            github.issues.addLabels({...context.issue, labels: ['status: planned']})

Add workflow that add the label planned when issue was planned to a milestone
Add workflow that add the label planned when issue was planned to a milestone
