Is there any PHP package that's very similar to the Plugin API? I really like the Drupal 8 plugin system. I am now also working on a non-Drupal related project and am wondering if there's any similar solution to that on packagist.org? Was trying to find one but without success (maybe wrong keywords).

Does anyone know where the idea was coming from? Or is it original Drupal thing?
Drupal 8 Plugin API can be installed as a standalone package like follows:
composer require drupal/core-plugin
To make use of it you will have to develop a bunch of integration code.
Unfortunately there is no much documentation on how to use Drupal components without Drupal core.