Persistent Notification for Calendar Events
I am replying (I see no one else has) because I have always used the calendar/notifications functionality in the EXACT same way you have, and was also distressed of this change in functionality.  I think we are in the minority, though, because your post is the first acknowledgement I have even seen anywhere about this issue.

The only solution I have is to look at third-party apps.  Although someone appears to have ported an older ICS/Jelly Bean-era version of the stock calendar which you can find in the Play Store, I have not used that too much.  At the moment I am using Business Calendar, which I THINK comes somewhat close to replicating the functionality you and I are accustomed to (although I am still watching it to see).  I also gave Today Calendar a very close look -- it is much closer to the Google aesthetic, and even has aspects of Material Design implemented.  It appeared also to have persistent notification functionality, although for some reason I had problems with the syncing.  This may have been fixed.

If you have come up with any solutions, I would love to hear them!  Good luck!