Registry Keys

Terminology

Not Installed - the ".xlam" or ".xla" file has been removed from the computer or shared drive.
Available - the ".xlam" or ".xla" file exists on the computer or shared drive.
Not Ticked - the add-in is available but has not been loaded in to Excel.
Loaded - the add-in is ticked and provides additional functionality.

Add-in Manager Key

The Add-in Manager key indicates that an add-in appears on the list in the Add-ins dialog box.
There can be entries here that point to files that are Not Installed.
This is updated when Excel closes.

Excel 365 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager 
Excel 2021 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager 
Excel 2019 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Add-in Manager 
Excel 2016 - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Add-in Manager 

Any add-ins that have been added under the "Options" key and then manually unticked by the user are removed from the "Options" key and added here.

Options Key

The Options key indicates that an add-in is installed and will be Loaded the next time Excel opens.
This is updated when Excel closes.
If an add-in has just been ticked by the user, the registry entry will be moved from the Add-in Manager part of the registry to the Options" part of the registry when Excel closes.

Excel 365 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options 
Excel 2021 - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Options 
Excel 2019 - HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options 
Excel 2016 - HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Excel\Options 

If the full path is not specified then the default folder location is "C:\Users\ "username" \AppData\Roaming\Microsoft\AddIns"

HKEY_LOCAL_MACHINE

HKEY_LOCAL_MACHINE\Software\Microsoft\Office\Excel\Addins\ 

Add-ins installed into local machine are also displayed in the COM add-ins dialog box.