The systemd-timesyncd daemon repeatedly tries to contact the following hard coded (compiled) time servers if neither an NTP server is specified manually nor one received via DHCP:
0.debian.pool.ntp.org
1.debian.pool.ntp.org
2.debian.pool.ntp.org
3.debian.pool.ntp.org
Since I'm operating behind a corporate proxy, it will never be successful to reach these servers. Is there a way to prevent this senseless polling (without recompilation)?
If you aren't allowed to contact any NTP servers to sync with, then disable the service that does that:
systemctl stop systemd-timesyncd
systemctl disable systemd-timesyncd
The first command will stop the service immediately. The second will prevent it from being restarted on the next boot.