This morning, when I did a flutter pub upgrade, GitHub returned the following error:

Git error. Command: `git fetch`)
stdout: 
stderr: fatal: remote error: 
  The unauthenticated git protocol on port 9418 is no longer supported.
Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
exit code: 128

A quick search led me to this ticket. This happens when a dependency uses an unauthenticated git url such as this:

  flutterfire_ui:
    git:
      url: git://github.com/FirebaseExtended/flutterfire.git
      path: packages/flutterfire_ui

Changing the protocol to https fixes it:

  flutterfire_ui:
    git:
      url: https://github.com/FirebaseExtended/flutterfire.git
      path: packages/flutterfire_ui