As you know, the Google Indexing API only is reserved for job posting URLs and live stream events. But that might change as we have been speculating for a while. Gary Illyes from Google in his Reddit AMA also mentioned that Google is testing this as well.
Gary Illyes wrote "Currently we're testing our own limitations with the indexing api as well as the usefulness of pushed content vs pulled. We don't really have anything to announce just yet."
This was in response to Yoast announcing integration with Google on submitting content to them. Gary added to that "As for Yoast, I don't want to step on anyone's toes (e.g. our Indexing API PM), but it may be that he over-announced some things."
We know Bing is onboard with pushed content over pulled content but it sounds like Google is not there yet.
Someone then asked Gary if Wix uses that indexing API and Gary said "As far as i know, yes, they're the only early testers. Though they're also making stupid ass statements about it..."
It would change a lot of the way SEOs think if people push content to Google vs let Google crawl and index that content.
Forum discussion at Reddit.