What is the difference between
More generally, what meaning does the
.d suffix convey to a directory?
¹ not counting initrd
The ".d" is usually appended to a directory name to indicate that what used to be (or what could have been) handled by a single script or a single configuration file has been split into multiple files for the sake of convenience, but which should be included, or executed, together.
For example, /etc/apache/conf.d/ or /etc/apt/sources.d/
In cases where it's important which order they should be included/executed, the files in these directories sometimes start with a number, like "00-default" or "80-user" so that they execute in the right order.
In the case of /etc/init.d/ it sort of indicates that the scripts in init.d should all be executed. Nowadays, however, the init system of modern operating systems is a bit more involved than that, but the directory name is still there.
As you point out, the ".d" nomenclature is puzzling and strange, and doesn't really have any place in any modern system -- you'll notice that most modern services have tended to drop it.
The reason the directory is