Starting from the Istanbul release, the 'CMDB Dashboard' is deprecated and replaced with 'CMDB View'. For instances upgraded from Helsinki or earlier, the CMDB Dashboard module will still be visible. In a base system Istanbul instance or newer, the 'CMDB View' is nested under CMDB Dashboard link, which is built on the newer Performance Analytics engine that standardizes the CMDB Dashboard with the rest of the ServiceNow platform. As some of the older metrics used by the older CMDB Dashboard are not available in Istanbul, we are unable to gather all of the information required for this other old CMDB Dashboard.
To access the CMDB View, please go to: CMDB Dashboard > CMDB View
There is also a system property which was added in Istanbul which will make the Dashboard 'responsive' and make visibility of the widgets much cleaner. To enable this, go to the below link and set that property to 'true':
We recommend unticking the 'Active' flag on the CMDB Dashboard module, which will hide it from the Application Navigator and avoid any confusion going forward - CMDB Dashboard: https://<INSTANCE_NAME>.service-now.com/nav_to.do?uri=sys_app_module.do?sys_id=b957e07ed71302002f6d69418e6103ac
Below the link to the official documentation - Monitor CMDB health in the CMDB dashboard: