That's actually still the case. `git` is just a wrapper that when called as `git <command>` looks for `git-<command>` under `libexec/git-core/`.

You can add any command named `git-foo` into your PATH and then run it with `git foo`. For example, I have `git-gsr` which is a simple shell script that does a global search and replace in a git repo. I run it as `git gsr <old> <new>`