The ms-appinstaller protocol has been disabled.

For anybody that is still having issues with this here is how I temporarily solved this problem.

When my application starts up it checks the version number (manually maintained in my App.xaml) against the version hosted on our webserver. If there is an updated version it downloads a zip file with msix and the generated shell scripts. It then upzips the files and runs the shell script through PowerShell to install the new version.

Note that this requires the device be in developer mode because executing the shell script is considered installing from an unknown publisher. I'm sure somebody smarter than me could find a work around that doesn't require developer mode (if so reply so I can make the changes).

This is completely unacceptable for an installation technology. MSIX team: if you’re not willing to properly support AppInstaller, please put it it out of its misery.

To anyone else facing implementing a workaround and is using Azure pipelines:

here is a yaml task that will edit the generated html to remove the protocol and include explicit instructions that the button will cause a download, and the downloaded file should be run.

Anonymized example from my implementation:

And here is the task:

    - task: PowerShell@2
      inputs:
        targetType: 'inline'
        script: |
          $x = gci -Recurse *.html
          $page = get-content $x
          $newpage = $page |  foreach { [regex]::Replace( $_,'ms-appinstaller\:\?source\=','' ) }
          $newpage = $newpage |  foreach { [regex]::Replace( $_,'(<div class.*app-description[^<]*</div>)','$1'+[System.Environment]::NewLine+'<div class="insert instructions"><p style="color:red" >Installation instructions:<br>1. Click the install button to begin download of installer file<br>2. After download completes run installer file to install application</p></div>' ) }
          set-content $x -Value $newpage

Note that there may be more efficient ways of accomplishing this, but I went with what I knew and could make generic.

Aditi_Narvekar - As MSIX is the flagship technology for deploying and updating Microsoft applications, I hope M$ is taking what amounts to a service outage as a very high priority. This issue has broken our entire devops workflow and is affecting user confidence in the solution. The additional manual steps required as a workaround at the moment are not appropriate for our user base.