So it’s an alternative to sitemaps, documents that can list incrementally all the web pages of a website with their last modification date-time, but with a push model instead of a pull model?
Yes, but of course only for the search engines that support it. Eg, Google is absent from the list, although they've already had a push/ping sitemap feature.
I thought so too, but search engines supporting IndexNow are supposed to tell each other about new pages, and there's a bit more of control since you can send just the urls you want instead of an entire sitemap.
If it was widely used it's win-win. More timely updates and less traffic crawling pages which haven't changed. That's if it ever becomes a reliable signal of page changes.