it should be available I believe.
on latest and on 6.4 also says the same thing -
Using custom alert actions
App developers can build custom, user-configurable, alert actions into their apps. Users can find apps with built-in custom alert actions from the alert actions manager page.
To try using a custom alert action, you can use the built-in webhook alert action to send notifications to a web resource, like a chat room or blog. For more information, see Use a webhook alert action.
To learn how to find apps with built-in alert actions, see Using the alert actions manager.
For more information on how to develop and use custom alert actions, see Custom alert actions in Developing Views and Apps for Splunk Web.