/lib/udev/rules.d/80-ifupdown.rules is in ifupdown 0.8.19.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 | # Allow rfkill for users in the netdev group
KERNEL=="rfkill", MODE="0664", GROUP="netdev"
# Handle allow-hotplug interfaces
SUBSYSTEM=="net", ACTION=="add|remove", RUN+="ifupdown-hotplug"
|