Indeed libssh is required for the sftp kio-slave. I decided not to add libssh as a dependency because of the comments I found about the quality of its code
Hmm, I hadn't come across libssh before. To be honest I'm not really
all that impressed with the code quality of libssh. There is a serious
lack of basic error checking in system calls & libc calls they make,
in particular no malloc() call is ever checked for failure. It is
hardcoded to use IPv4 for sockets. It is not const-correct in its
API usage, or its exposed public API.
Perhaps this library will mature with the passing of time, but since it will be responsible for your encrypted sftp connections in KDE, I decided to skip its addition for now.