/usr/lib/x86_64-linux-gnu/libssh_threads.so is in libssh-gcrypt-dev 0.7.3-2+deb9u2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libssh-gcrypt_threads.so.4
/usr/lib/x86_64-linux-gnu/libssh-gcrypt_threads.so.4