Terraform cannot include PowerShell autocompletion sort because they use your library and they believe your library does not offer support for PowerShell completion.
From TerraForm GitHub issue:
Terraform's tab-completion is powered by the external library github.com/posener/complete, so we support the same set of shells which that library supports. Therefore, if you send this request to that library or propose a PR that they accept then we can upgrade our version of that library to inherit that PowerShell support.
Apparently a rival completion library called Cobra does offer support for PowerShell.
Terraform cannot include PowerShell autocompletion sort because they use your library and they believe your library does not offer support for PowerShell completion.
From TerraForm GitHub issue:
Apparently a rival completion library called Cobra does offer support for PowerShell.