Damn, you really got me scared there for a second. Just re-checked by Debian 12 systems on AWS, I don't have any files residing in *.d directories. So I confirm that if you used the official AWS Debian image, you wouldn't get it.
And if you downloaded and installed the authentic Debian 12 image from debian.org, you don't get it either. Must be a Ubuntu thingy.
Great, now you can start putting your customizations in that directory instead of the OS-managed /etc/ssh/sshd_config blob. That's why the `.d/` convention exists.
I had to look up the info because I wasn't sure really, so I appreciate the direct question. It means 'directory' or 'conf.d pattern'. So you have your config file, and the config directory that contains "parts" of the config to be merged with the main one to provide customizations/overrides.
