Microsoft (Bing) and Yandex have defined a simple interface (API) with IndexNow via which website operators can use to inform search engines about new, changed or deleted URLs.
IndexNow is explicitly not an indexing API: it is not the content of the website that is sent via the interface, but only the URL. The crawler of each participating search engine still has to call, parse and process the URL.
The fact that Google, with its market share of well over 90 percent, does not support IndexNow shows that the new interface does not solve any real problem. The detection of new or updated URLs has been possible for many years via XML sitemaps; no new standard is required for this.
On the contrary, some time ago Google operated a comparable service via its own blog ping server, which was notified by WordPress when new blog posts were published – and discontinued it.
The real problem with search engines is not to recognize as many new or changed URLs as possible, but the correct ones, and that cannot be solved via a public API without access control.