I'm not 100% sure, but I think
Citynightline stopped using the double-decker carriages as of the timetable change in December 2014 (at the same time as sadly cutting all services to/from Paris and Copenhagen). So, I think all remaining services are using the Comfortline sleepers. There is a list
here - which doesn't show Milan-Munich at all, but I think that service is still running (presumably also using Comfortline).
ÖBB also had a number of double-decker sleepers - so if they're still using them, you might be able to travel on one of theirs - but I don't know which routes they're on. You might be able to work it out from the
vagonweb site.