/usr/lib/x86_64-linux-gnu/libspatialindex.so is in libspatialindex-dev 1.8.5-5.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libspatialindex.so.4.0.1
/usr/lib/x86_64-linux-gnu/libspatialindex.so.4.0.1