This is part of the more general concept of “markdown extensions”.
Your definition of “plugin” is unnecessarily strict, in my opinion.
There shouldn’t need to be just one “plugin syntax”.
I would want to see something like this:
This Markdown Extension adds Hugo Shortcodes.
A shortcode is an inline element, and is parsed inside of both block and inline HTML.
A shortcode begins with two opening braces …
There’s some discussion over here about extensions:
My contribution to the topic is basically reproduced above - basically stating that extensions should have specs similar to the Standard Markdown spec.