/etc/pam.d/proftpd is in proftpd-basic 1.3.5-1.1+deb8u2.
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 6 7 8 9 10 11 | #%PAM-1.0
auth required pam_listfile.so item=user sense=deny file=/etc/ftpusers onerr=succeed
@include common-auth
# This is disabled because anonymous logins will fail otherwise,
# unless you give the 'ftp' user a valid shell, or /bin/false and add
# /bin/false to /etc/shells.
#auth required pam_shells.so
@include common-account
@include common-session
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