The clash check feature in the browser will move from the legacy 3D viewer to the new Connect 3D Viewer during January 2020. This means that the "Clash sets" panel will stop being available in the legacy 3D Viewer and become available in the new 3D Viewer instead. Clash sets created with the legacy viewer will continue to be available in the new 3D Viewer. Also, the clash set panel will continue to be available in the Trimble Connect for Windows application.
This change happens as part of the deprecation of the legacy viewer. Please see more information here: