I just spent the whole morning trying to figure this out. I don't understand it!!!
I currently have a LibreELEC (kodi OS) and it lists the VFS add ons. If I install kodi in Ubuntu via the kodi repo, I also can apt-get kodi-vfs-fstp and then it is listed in the add ons. This also works via an Ubuntu based Raspberry pie setup.
All that said, recalbox has been the best for actuall retro gaming, but I need to be able to add both sftp and a web browser to kodi.
As I said, this works fine via LibreELEC, but kodi gaming sucks. I have been unable to load the add ons. I found the zip file downloads, but they ALL gave the same error. Somthing about the wrong layout or somthing.
way down at the bottom, vfs.sftp+... https://mirrors.kodi.tv/addons/leia/