/usr/share/lintian/overrides/apt-watch-backend is in apt-watch-backend 0.4.0-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | #apt-watch-auth-helper authenticates the user with pam, so it must be setuid
apt-watch-backend: setuid-binary usr/lib/apt-watch/apt-watch-auth-helper 4755 root/root
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