/usr/lib/libdeal_II_2d.so is in libdeal.ii-dev 6.3.1-1.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libdeal_II_2d.so.6.3.1
/usr/lib/libdeal_II_2d.so.6.3.1