/usr/lib/x86_64-linux-gnu/libnotmuch.so is in libnotmuch-dev 0.21-3ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libnotmuch.so.4.3.0
/usr/lib/x86_64-linux-gnu/libnotmuch.so.4.3.0