Weird, Sierra works fantastically on my mid 2010 cMP. (Haven't tried the night shift update).This is the unfortunate truth. I'm still using El Capitan with my Mid 2011 Imac, even though Sierra was introduced just five years after the machine was built (September 2016). I haven't been bothered to run the update.
Almost none of the new features of Sierra work on on a Mid 2011 machine. At first I figured that they had to throw us a bone with Night Shift, as there was no technical reason to deny it. However, apparently they're not going support a five year-old machine with OS features anymore. Having used Imacs for a long time, this is the fastest I've seen them try to make a particular machine obsolete in the post PPC era.
Either way, obsolescence is an ever-changing idea. If I was stuck on my cMP for 3 more years I think I'd be fine (maybe I'll flash a new graphics card, or put 64gb of ram in, but that's all). So honestly running El Capitan on a 2011 iMac doesn't sound too bad to me, especially when many people I know refuse to update to Windows 10 and still Run 7.