It appears as though Esri is phasing out its application-agnostic industry data models for solution-specific data sets. In the Technical FAQs on Esri's site, under "Does Esri have industry-specific data models?", it states that:
Note:This article will soon be retired. For the most current Esri data model information, navigate to ArcGIS Solutions or the Solutions Gallery to access industry-specific configurations for ArcGIS along with ready-to-use maps and apps to jump-start your work. In addition, use the ArcGIS Solutions Deployment Tool to browse and deploy hundreds of apps dynamically managed by Esri.
What is happening with the data model user groups? The link to join a data model user group currently (2/1/2020) returns a 404 error.
