/etc/apache2/suexec/www-data is in apache2-suexec-custom 2.4.10-10+deb8u12.
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 | /var/www
public_html/cgi-bin
# The first two lines contain the suexec document root and the suexec userdir
# suffix. If one of them is disabled by prepending a # character, suexec will
# refuse the corresponding type of request.
# This config file is only used by the apache2-suexec-custom package. See the
# suexec man page included in the package for more details.
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