Sometimes you need to do things you don't like and you don't know where you will end up.
In our household there exists one (production) system running Windows. Don't ask why and please no recommandations how to substitute it. Some things are hard to (ex)change, for example your love partner.
Looking into Backup with rsync on Windows (WSL) I needed to start a privileged powershell, so I first started an unprivileged one:
powershell
Just to start a privileged:
Start-Process powershell -Verb runAs
Now you can follow the Instructions from Microsoft to install OpenSSH. Or just install the OpenSSH Server:
Add-WindowsCapability -Online -Name OpenSSH.Server~~~~0.0.1.0
Check if a firewall rule was created (maybe you want to adjust it):
Get-NetFirewallRule -Name *ssh*
Start the OpenSSH server:
Start-Service sshd
Running OpenSSH server as service:
Set-Service -Name sshd -StartupType 'Automatic'
You can create the .ssh directory with the correct permissions by connecting to localhost and creating the known_hosts file.
ssh user@127.0.0.1
When you intend to use public key authentication for users in the administrators group, have a look into How to Login Windows Using SSH Key Under Local Admin.
Indeed you can get rsync running via WSL. But why load tons of dependencies on your system? With the installation of rsync I cheated a bit and used chocolately by running choco install rsync, but there is also an issue requesting rsync support for the OpenSSH server which includes an archive with a rsync.exe and libraries which may also fit. You can place those files for example into C:\Windows\System32\OpenSSH so they are in the PATH.
So here we are. Now I can solve all my other issues with BackupPC, Windows firewall and the network challenges to get access to the isolated dedicated network of the windows system.