IndexNow is a protocol jointly developed by Microsoft Bing and other global search engine companies, including Yandex. Its purpose is to easily inform search engines about changes to websites. Specifically, IndexNow notifies search engines when a webpage is published, deleted, or modified.
There were articles around the end of 2021 suggesting that Google was testing the IndexNow protocol. However, there seems to be no official support yet. But, using the Index Now plugin mentioned above, you can activate Google's Indexing API to notify Google's search engine about web page additions or modifications.
Applying Google's IndexNow to WordPress
Although there were rumors in the last two years that Google was testing the IndexNow protocol, there doesn't appear to be any official announcement about Google supporting IndexNow yet.
However, the Index Now plugin does have a tab for the Google API along with the Bing API.
If you search for "Google IndexNow API" on Google, you won't find relevant results. It seems that it uses a different method, specifically the Indexing API, to request indexing from Google.
By creating and inputting the Google JSON key, the Google IndexNow feature will be activated. The process to create the Google JSON key can be a bit complex. Instructions are provided when you click on the question mark next to the Google JSON Key.
It looks like you create a service account from Service Accounts. The actual screen in the Google Cloud Platform may differ from the instructions. I had a bit of difficulty finding the Service Accounts menu. Searching for Service Accounts in the search field should help you locate it.
After following the manual instructions to create the JSON key and activate the Indexing API, it seems to work well.
However, there might be times when you receive an error even if you follow the instructions:
Indexing API has not been used in project YOUR_PROJECT_ID before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/indexing.googleapis.com/overview?project=YOUR_PROJECT_ID then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.
In such cases, activating the Indexing API by clicking on the link should solve the issue.
In conclusion
For Google, automatic indexing usually works well, so there doesn't seem to be a need to make crawling requests using this method. However, if Google is not indexing content from my site, using this method can be helpful for sending index requests when posts are published or when existing articles are modified.
The actual effect will need time for evaluation. I have been notifying the Bing search engine every time I publish new content through the IndexNow protocol, but the effect doesn't seem significant. This time, I've set it up to send IndexNow requests to Naver, a Korean popular search engine, and Google. I plan to monitor its effectiveness over a few months.