GitHub actions to create the upm branch. https://medium.com/openupm/how-to-maintain-upm-package-part-1-7b4daf88d4c4#4af4

ci.yml

name: CI
on:
  push:
    branches:
    - master
jobs:
  split-upm:
    name: split upm branch
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v3
      with:
        fetch-depth: 0
    - name: split upm branch
      run: |
        git subtree split -P "$PKG_ROOT" -b upm
        git push -u origin upm
      env:
        PKG_ROOT: Packages/package-name