/usr/lib/x86_64-linux-gnu/libnotmuch.so is in libnotmuch-dev 0.26-1ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libnotmuch.so.5.0.0
/usr/lib/x86_64-linux-gnu/libnotmuch.so.5.0.0