Installing OpenSSH on Windows 10 which had been upgraded from Windows 7

When trying to install OpenSSH on a machine running Windows 10 which had been upgraded from Windows 7, the Optional Features under Apps -> Apps and Features -> Manage Optional Features as mentioned in the documentation was completely blank. But the install with Powershell (run as administrator) method as mentioned in the same documentation page worked.
Add-WindowsCapability -Online -Name OpenSSH*

For troubleshooting permission issues, this post or this one may be useful - it talks about using PowerShell to change the permissions - ACL - on the authorized_keys file. Or, just make sure the permissions are appropriate using windows explorer -> file properties -> security -> advanced, 

For the private key (and for the authorized_keys file?), 

change the owner to the login user (if it's not already).

Disable inheritance (if it's set).

Remove all permissions for every one but this user.

Give this user “Full Control”.