Aren’t there environment variables you can view on the other platforms for the OS?
Get-ChildItem -Path Env:
Particularly, on Windows at least, there’s an OS environment variable, so you should be able to accomplish this by using $Env:OS.
Since some time has passed and the PowerShell Core (v6) product is GA now (the Core branding has been dropped as of v7), you can more accurately determine your platform based on the following automatic boolean variables:
$IsMacOS
$IsLinux
$IsWindows