/bin/debconf-set is in ubiquity 2.21.63.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 | #!/bin/sh
set -e
export DEBIAN_FRONTEND=noninteractive
. /usr/share/debconf/confmodule
db_set "$1" "$2"
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