Fresh www-servers/apache-2.2.21-r1 install contain in /etc/apache2/modules.d/00_default_settings.conf this:
# To add files to that list use AddDirectoryIndex in a custom config
# file. Do not change this entry unless you know what you are doing.
DirectoryIndex index.html index.html.var
AFAIK AddDirectoryIndex is deprecated (see bug 139451 for example) and doesn't even mentioned in current apache docs, so this comment in config file should be changed or removed.
Confirming that AddDirectoryIndex hasn't worked for a while. The comment can certainly go, but I wonder if we even need the DirectoryIndex line at all. "DirectoryIndex index.html" is the default in at least 2.2 and 2.4. I don't know what the index.html.var is supposed to accomplish.
This will be in the next gentoo-apache tarball rollout. Please keep this bug open until a fixed apache is in portage.
Looks like it's still valid with 2.4.18 even :(
The above mentioned change has been rolled out for quite a while already.