/usr/lib/x86_64-linux-gnu/libcinnamon-desktop.so is in libcinnamon-desktop-dev 2.8.0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libcinnamon-desktop.so.4.0.0
/usr/lib/x86_64-linux-gnu/libcinnamon-desktop.so.4.0.0