Add data alter hook to cope with multiple handlers

This is a good idea.
Some feedback:

1) seems a bit silly to have both those in succession, though i understand the instinct to preserve existing functionality. Let's mark the existing hook deprecated, and to be removed in 5.0 stable release. And let's fire a watchdog error for anyone still using it, so they know to replace their impelmentations.

2) let's replace it with an event dispatcher, instead of a hook. Let me know if you want help on figuring what that event should look like.

Looks good so far.

A couple minor points real quick:

+    $salesforce_data_object = (object) $salesforce_data;
+    /** @var \Symfony\Component\EventDispatcher\EventDispatcherInterface $dispatcher */
+    $dispatcher = \Drupal::service('event_dispatcher');
+    $event = new Sfweb2leadWebformEvent($salesforce_data_object, $this, $webform_submission);
+    $dispatcher->dispatch(Sfweb2leadWebformEvent::SUBMIT, $event);
+    return (array) $salesforce_data_object;

There's no reason to cast $salesforce_data to an object, then back.
It's a property of Sfweb2leadWebformEvent, which will be passed by reference - expose it via setter(s) instead:

+    /** @var \Symfony\Component\EventDispatcher\EventDispatcherInterface $dispatcher */
+    $dispatcher = \Drupal::service('event_dispatcher');
+    $event = new Sfweb2leadWebformEvent($salesforce_data, $this, $webform_submission);
+    $dispatcher->dispatch(Sfweb2leadWebformEvent::SUBMIT, $event);
+    return $event->getData();

And add a typehint to the constructor like so:
+ public function __construct(array $data, SalesforceWebToLeadPostWebformHandler $handler, WebformSubmissionInterface $submission) {

I'm going to fix the above minor points.

Here we go

Thanks for the patch. I've committed a slightly updated version.