From #8269 (comment)
@DeepDiver1975 I checked the sample file on my 7.0.3 installation and https://github.com/owncloud/core/blob/master/config/config.sample.php in the repo. I couldn't find any reference to index.php.
Looks like the config flag is not documented - sorry for this. The config setting is named 'front_controller_active' and it should be set to true.