It’s an old thread, but since I came across this while looking for something else, I may as well give a workaround for this one. At least there may be others on Windows who may benefit from this.
OpenSSH has been part of Windows installation at least since Windows 10. So, you can basically make an ssh connection to your host from the command prompt: ssh username@example.com. Immediately after that you will be asked something like:
The authenticity of host 'example.com (93.184.216.34)' can't be established.
ECDSA key fingerprint is SHA256:VHJ1c3QgbWUsIEkga25vdyB3aGF0IEknbSBkb2luZy4.
Are you sure you want to continue connecting (yes/no)?
After you write yes, the public key of the host will be saved into the file
C:\Users\username\.ssh\known_hosts. The host will ask for your password after that, but you don’t have to log in at this point if you don’t want to.
Now, you can use that known_hosts file as the one Kopia requires for the SFTP connection. And if you have multiple hosts you want to use for backups, you can just do the same with each one of them, and all their keys will be saved into the same known_hosts file.