Occasionally I find myself needing plugin-like functionality, where users/downstream can throw a class into a folder and expect it to work. My script is supposed to find and instantiate these plugins during runtime without keeping track of their existence.
In a regular Drupal module, one would usually use the plugin architecture, but that comes with its overhead of boilerplate code and may not be the solution for the simplest of use cases.
Many class finder libraries rely on get_declared_classes() which may not be helpful, as the classes in question may not have been declared yet.
If you are on a Drupal 8/9 installation and want to use components already available to you, the Symfony (file) Finder can be an alternative for finding classes in a given namespace.
Installing dependencies
Ouside of Drupal 8/9, you may need to require this library in your application: