Why is it so hard to find information about this subject?

This is not a roadmap from Sage for API but you can look at http://www.sagedataexchange.com/ which is a 3rd party modification that would give you what you are looking for. 

As to what Sage has planned, As far as a know the existing Web Services (eBM) and sData are being supported and released with each version update but they have no planned development to change or enhance them. BOI is the primary interface that you would have. They do have an Office 365 Integration which would allow you to connect through Power Apps and power automate. This allows you to have a custom connector. This is something that they are currently developing and is on the Roadmap for integrations. 

Sage is building an Office 365 integration (cloud based connector to on premise data)... I've seen some demos and it has extremely powerful potential.

https://help-sage100.na.sage.com/2019/index.htm#cshid=701002&chm=LM

We had the original O365/Sage Contact up and running.  The problem we ran into is the MS Office 365 demo site would only last for 90 days, thus blowing away our work.  We used it for a client event to demo a day in the life of someone using Sage 100 and Outlook.  But we were not into having to re-provision a demo site every 90 days.  And finding helpful documentation for the API's that have been demonstrated at various summit's and Sage events is very difficult.  

Yes Documentation for anything that is custom like this is very limited, unfortunately.They have done better (still not great) with things like Customizer which more end users would use but things like Web Services, sData in Sage 100 (not including the sData github site which had a lot of info), BOI it is light in what is available.