Discover and assign the Microsoft 365 Extra Features license

mikezapp - we are glad this information helps. Regarding your questions: 

Question 1 Is there a way to prevent a new "extra" service plan added from being enabled by default? 

You may follow this guide: Disable access to Microsoft 365 services while assigning user licenses - Microsoft 365 Enterprise | Microsoft Learn

Per VasilMichev suggestion, we are working to add the identifiers.

Question 2 - will this post of all extra features and links to their description be maintained or is there a Microsoft source somewhere else to use as an ongoing resource that will be maintained as the extra service plans increase?

We plan to update this page or the ms learn article: Understand the Microsoft 365 E3 Extra Features license | Microsoft Learn