5.1 on Windows 10. Admittedly, I never really tried iwr on older versions. Heck, the last time I tried downloading something with PowerShell to a file I went the WebClient.DownloadFile route, so it's been a while.
You can probably install a more recent version of WinRM on that machine, though (or by now probably the open-source version of PowerShell).
I'm doing this on windows 2008 server.