Hi @Timothy_Smith,
Let me say the following, the API is in an absolute mess now-a-days. There is no communication about things, no Roadmap you can see, lack of progress, not listening to the community, and usually very little official response when anyone ask questions about it.
As far as I know the API started its life in 2006 under the XML-RPC API. That progress quite well over the years until the company decided it was no longer suitable for future expansion. In the latter part of 2016 the REST API was introduced with new Endpoints slowly being added. You would see people several years ago saying that XML-RPC would be deprecated in favour of the REST API. That is understandable progress needs to be made, and we would have to make the changes to do so. In a perfect world, or in other words, people who know what they are doing, we should all be using the REST API by now.
Unfortunately, bad decisions have been made along the way. The REST API v1 was not comparable with the XML-RPC API, as it misses several pieces of functionality. To address that issue the company decided to work on REST API v2 to overcome the shortfalls. But from what we are seeing, it is still not comparable with the XML-RPC API. The REST API Querying capabilities are awful and are not matching the XML-RPC API. In a recent discussion the company mentioned they will not offer the ability to search Custom Fields because they are not Indexed.
What has happened is now we have integrations that are either using the XML-RPC, or REST v1, or REST v2, or combinations of them.
Here is a recent example of someone asking a REST API question 3 years ago, and still no official response to it. You have to use the XML-RPC to do that task.
The only thing I know that is coming is that the PIpeline API being developed which was recently mentioned in this post.
Who knows what is going to happen in the future. Maybe by the year 2030 the REST API v2 will be completed!