That exists under the hood since long time: https://blogs.msdn.microsoft.com/sharepointdeveloperdocs/2007/12/07/customizing-alert-notifications-and-alert-templates-in-windows-sharepoint-services-3-0/
Undust it, add access via Client side APIs?!
69 votes3 comments · Sites and Collaboration » Workflow, Flow, and PowerApps integration · Flag idea as inappropriate… · Admin →
Essentially: Adding REST Endpoints to Word Automation Service and also bringing it to SPO would do it!
1,527 votes31 comments · SharePoint Dev Platform » SharePoint APIs (CSOM/REST/Server-side) · Flag idea as inappropriate… · Admin →
Thanks for your feedback! Just a note to let you know that we’re building your suggestion now. We do apologize for the delay on getting this moving, but we are now working on exposing term store operation properly with REST API/Microsoft Graph. We will be covering any updates on the API status, before the actual release, in the SharePoint Dev community calls. Thanks, everyone for your input around this capability.
Ok, good to read that MSFT made that happen. Though: Does the Solution file being on Github mean I would not get any Premier Support for that case in case I'd need some? (for FIM scenarios, as powerful as they were sometimes that was not avoidable)