It has an issues tracker. Feel free to add your comments, but I don’t see much recent code change activity. Duplicati at least keeps changing code even though the issues list sometimes seems to have no bottom… Duplicati’s lead author did do AES in C#, but I don’t know if he or anyone else here can add new KEX code.
No secure HMAC algorithms offered for SSH/SFTP seemed to say that obviously the library gets replaced, however it offers no specific suggestions for something compatible with open source. I’m not finding much.
Key Exchange (KEX) Method Updates and Recommendations for Secure Shell (SSH) is an effort to update IETF (Internet Engineering Task Force – a.k.a. the global standards body for such things) advice about KEX algorithms. Interestingly, they still recommend diffie-hellman-group14-sha1 (for now, due to availability problems for better algorithms) and so does SSH.COM. So that might be the best of what SSH.NET can do.
Probably your perspective also varies depending on whether you’re a server who has to let popular clients connect somehow (or suffer complaints) or someone seeking a client which is ahead of the broader pack.
SSH implementation comparison tries to track who has what, though I don’t know how current the data is.
I haven’t verified it, but I think what Duplicati has is limited to the below, though note others are comments:
By comparison, I think rclone uses the following, and this looks like it knows (maybe even supports) more:
https://github.com/golang/crypto/blob/4def268fd1a49955bfb3dda92fe3db4f924f2285/ssh/kex.go#L22