The Associate and Disassociate messages are still supported for Crm 2013, and the method signatures haven't changed, so I would expect this to still work.
Are you sure the plugin is not firing, or is it the case that you have logic based on the IPluginExecutionContext, and this logic is now working differently ?
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk