There doesn't seem to be a place that describes the policy. That API-object, documentation page would be perfect for it.
However, a lead Stack Exchange developer has stated that anything that reveals voting patterns will be hidden or private.
Another Stack Exchange employee explains how user deletions and post deletions are kept private-ish, but ...
The comments are more or less correct; caching applies to the /events endpoint just like every other method.
This is by design for now. We have looked at differing caching policies for /events and /inbox/* routes, as the minute lag on those methods is a bit heavy handed; though we still need to protect ourselves from abusive requests.
There is currently no method to filter events by event_type.
For now, just load all the events and filter through the event_type values afterwards. Since the /events route only covers the last 15 minutes, it will only take 2 to 10 pages to get all the unfiltered results on Stack Overflow (the busiest site).
However, your options increase depending on what ...