/usr/lib/x86_64-linux-gnu/libmodbus.so is in libmodbus-dev 3.0.5-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmodbus.so.5.0.4
/usr/lib/x86_64-linux-gnu/libmodbus.so.5.0.4