/usr/lib/x86_64-linux-gnu/libQtAssistantClient.so is in libqtassistantclient-dev 4.6.3-7+b1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libQtAssistantClient.so.4.8.7
/usr/lib/x86_64-linux-gnu/libQtAssistantClient.so.4.8.7