Can you assign Hide TOC On Enter to all Master slides?

Maybe better use the JS script posted above, because it is not possible to assign an action On Enter for a Master slide.

All depends how you create the 'template' because I see a lot of confusion between 'theme' and 'template'. It is pretty easy to assign 'Hide TOC' to the On Enter event of all slides: just select all slides in the Filmstrip with CTRL-A and assign the action Hide TOC to the On Enter event. If template means that the users always have to use copies of slides with placeholders, that will do the trick.

I do this with the common JS Interface.You can put this in a JS file or in the head of the html page. If html enclose in <script></script> tags.

var interfaceObj, eventEmitterObj;

window.addEventListener( "moduleReadyEvent", function( e )
{
interfaceObj = e.Data;
eventEmitterObj = interfaceObj.getEventEmitter();
    initializeEventListeners();
});

function initializeEventListeners()
{
if ( interfaceObj )
{
     if ( eventEmitterObj )
  {
         eventEmitterObj.addEventListener( "CPAPI_SLIDEENTER", function( e )
   {
    window.cpAPIInterface.setVariableValue( "cpCmndTOCVisible", 0 );  
            });
  }
}
}