libcamera relies on 2 environment variables to decide what to log, and where: LIBCAMERA_LOG_LEVELS and LIBCAMERA_LOG_FILE (see here).
Passing values for these in the app's execution environment will let you control the looging. The easiest way to do it is to modify the desktop file.
For SailfishOS, that is harbour-shutter.desktop, for Ubuntu Touch click/harbour-shutter-ui.desktop
The following works on Ubuntu Touch ONLY, and will prevent the app from launching on Sailfish.
Change the Exec=harbour-shutter line to Exec=env LIBCAMERA_LOG_LEVELS='*:DEBUG' env LIBCAMERA_LOG_FILE="/tmp/libcamera.log" harbour-shutter.