[jonathan_ou]

Still no solution, just deafening silence....

In case that can help someone: After also wasting a few hours of my life, I eventually managed to get sftp/ssh sources with key pair authentication working, both on Libreelec and OSMC.

What eventually did the trick was to wipe out the content of .ssh/known_hosts (take a backup just in case).
I didn't bother looking at the content difference, but the answer is probably there.

Along the way, I noticed that the sources.xml syntax for sftp has also changed a bit.

It went from:
<path>sftp://username:password@Server.com/share/videos/</path>

To
<path>sftp|ssh://username:password@Server.com//share/videos/</path>

Both still seem to work at the moment, but I guess it will be more future proof to update it. 

Deleting .ssh worked for my LibreElec, but not for my Macbook.  Any ideas why Kodi 18 on Mac can't use sftp?  I'm running MacOS 10.13.6.

Here's my relevant kodi.log info: https://paste.kodi.tv/uyayosivoh.kodi