3D Scene > Materials option 'Purge Unreferenced' to be enabled by default
Materials referenced only by deleted models will now be removed by default to help reduce file size.
Though the 'Purge Unreferenced' option has long since been available, it is now enabled by default (though can be disabled for each 3D layer) to avoid the situation where unnecessary materials are left around in saved drawings, which would otherwise bloat the project and database
See also: Impact Explorer 3D Scene tab