/usr/share/postgresql/9.5/pg_service.conf.sample is in postgresql-9.5 9.5.14-0ubuntu0.16.04.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Connection configuration file
#
# A service is a set of named connection parameters. You may specify
# multiple services in this file. Each starts with a service name in
# brackets. Subsequent lines have connection configuration parameters of
# the pattern "param=value" or LDAP URLs starting with "ldap://"
# to look up such parameters. A sample configuration for postgres is
# included in this file. Lines beginning with '#' are comments.
#
# Copy this to /etc/postgresql-common/ (or select its location with the
# PGSYSCONFDIR environment variable) and rename it pg_service.conf.
#
#
#[postgres]
#dbname=postgres
#user=postgres
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