This is a non comprehensive list of what is required to deprecate master branch:

Blocking switch for flutter/flutter

• Update the flutter.dev website
• Mirror master branch to main
• Validate full support for main alias in flutter tool

Switch time

1. Block PR submission
2. Select new default branch in repo settings
3. Re-target all open PRs towards main
   From what I gather, since the old branch will remain, GitHub will not retarget PRs automatically, so this will likely best be achived with the CLI gh
4. Change trigger branch from master to main for presubmit checks and workflows
5. Flip mirroring direction from main to master
6. Open PR submission

Other non-blocking issues

• Migrate existing SDK users on the master channel/branch to main
  [flutter_tools] migrate Flutter SDK users on the master channel to main #121565
• Update frob tooling to use main
• Update https://github.com/flutter/cocoon
  Update link branches to main cocoon#3671
• Set up main branch as default development branch devtools#7551
• Set up main branch as default development branch dart-lang/test#2209
• Set up main branch as default development branch flutter-intellij#7342
• Update conductor to use main
• Validate full support for main alias in analytics
• Update master references across flutter, engine and website
  Reland: Update link branches to main #146882
  Update link branches to main (continued) #146985
• create main-docs-flutter-dev gcp project and update all the workflows to publish docs there master-docs-flutter-dev
• Update master references in api docs and/or samples
• Update master docs URL from https://master-api.flutter.dev to https://main-api.flutter.dev
• Update DartPad.dev
• Update https://github.com/flutter/platform_tests to use main
• Update https://github.com/flutter/flutter_gallery_assets to use main