众所周知， debian系列的软件源列表, 从最早手动的改动
甚至 例如 fail2ban的配置文件 已经明确说了, 还是有人在改主conf
# HOW TO ACTIVATE JAILS:
When distribution packaging became more and more common, it became clear that we needed better ways of forming such configuration files out of multiple fragments, often provided by multiple independent packages. Each package that needs to configure some shared service should be able to manage only its configuration without having to edit a shared configuration file used by other packages.
The most common convention adopted was to permit including a directory full of configuration files, where anything dropped into that directory would become active and part of that configuration. As that convention became more widespread, that directory was usually named after the configuration file that it was replacing or augmenting. But since one cannot have a directory and a file with the same name, some method was required to distinguish, so .d was appended to the end of the configuration file name. Hence, a configuration file /etc/Muttrc was augmented by fragments in /etc/Muttrc.d, /etc/bash_completion was augmented with /etc/bash_completion.d/*, and so forth. Sometimes slight variations on that convention are used, such as /etc/xinetd.d to supplement /etc/xinetd.conf, or /etc/apache2/conf.d to supplement /etc/apache2/apache2.conf. But it’s the same basic idea.
Generally when you see that *.d convention, it means “this is a directory holding a bunch of configuration fragments which will be merged together into configuration for some service.”