The use of the Google Indexing API is theoretically limited for certain types of pages containing specific schemas.
However, many users and experts have observed good results using it for any type of content. That being said, it’s more of a hack. It may not work for everyone or Google will reduce its use.
Regarding the IndexNow API used by Bing / Yandex (and maybe Google in the more or less near future, they have announced that they are studying this possibility), everything is official and you have every interest in using it.
Note that it doesn’t replace the use of a XML sitemaps.