Hooks are invoked and events dispatched using an arbitrary string. Hook implementations and event listeners then get picked up based on that arbitrary string.
However, with events, we've adopted the pattern of putting the string in a constant on an Events class. See for example https://api.drupal.org/api/drupal/core!modules!migrate!src!Event!Migrate...
This has caused problems on module enable, see #2712647: Update Symfony components to ~3.2 → and #2776235: Cached autoloader misses cause failures when missed class becomes available → .
Event listeners reference the event class in getSubscribedEvents() - but they either have to rely on the event class always being loaded, or use class_exists() to check if it's available. This introduces a very fragile dependency on the classloader being updated when the module list is.
Stop using event classes with constants for event names, just use strings everywhere.
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And $classEvent::EVENT_1, $classEvent::EVENT_2 when there is more than one thing using the event (though the original pattern might have been there for discoverability) OR just create a base class and use ::class anyway.
I think we should avoid using an event class for many events. Each event should have its own class, even that would need a base class or a trait to handle core reusability
