We are deploying a number of AddIns for ArcMap. Should be ESRI.ArcGIS.Desktop.AddIns.dll included in every *.esriAddIn?
For development, C:\Program Files (x86)\ArcGIS\DeveloperKit10.6\bin\ESRI.ArcGIS.Desktop.AddIns.dll is used.
C#, ArcMap 10.6.1
