Solution
Starting from VeriStand 2021, the NI-maintained Custom Devices (such as the Instrument Addon, Embedded Datalogger, or XCP or CCP Master custom devices) are installed in a separate Add-On package called "VeriStand Custom Devices". You can install this Add-On from NI Package Manager, by searching VeriStand Custom Devices or you can download it here.

This will add the Custom Devices mentioned in the Overview tab to the appropriate file directory and will make them available in the VeriStand project.
If you are using custom devices that you have built yourself or custom devices that are not maintained by NI, you will need to rebuild them from the source code for your current VeriStand version. Refer to the VeriStand Custom Devices Handbook for more information.