/etc/default/icinga2 is in icinga2-common 2.6.0-2+deb9u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # default settings for icinga2's initscript
#DAEMON_ARGS="-e /var/log/icinga2/icinga2.err"
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