ArcObjects SDK 10.8 is not available as a public standalone download. Because it is part of the retired ArcGIS Desktop product line, you must access it through official Esri licensing portals. Geographic Information Systems Stack Exchange How to Access the SDK My Esri Portal : Log in to the My Esri Customer Service portal using a registered global account. Navigate to My Organizations
Log in with a global account associated with your organization's license. Navigate to the tab. Locate ArcGIS Desktop or ArcGIS Engine .
: Click the download button to receive the self-extracting executable file. System & Installation Requirements
, which uses modern .NET (Core/6/8) and a multi-threaded architecture, unlike the COM-based ArcObjects. If you'd like, I can help you with: Troubleshooting