You are correct that incident acknowledgements are not treated as events when using either the API or the Alerts UI. Although you can see them in the Alerts UI (if an incident has been acknowledged, it will show the user who did it and the time of acknowledging in the top-right corner), there is no way to access these using the REST API.
That said, our Product Managers and Engineers who plan our product roadmap are always interested in hearing new ideas about how to make New Relic even better, so I have submitted this as a feature request to be considered for a future release. We’ll do our best to let you know if this is implemented. While we can’t guarantee when or if this feature will be implemented, we take customer requests very seriously and use them to prioritize which features we implement next.
If you have an opportunity, please visit the Feature Ideas section (https://discuss.newrelic.com/c/feature-ideas/feature-ideas-alerts) where you can add your use case for the proposed feature as a Feature Idea. This will allow other New Relic users to discover your idea and participate in the conversation by adding their use cases, possible workarounds, and even their vote.
Thanks for helping us improve the product!