There are instructions on how to install macOS Sierra on your iMac, so it is possible. Although I reckon it'll get more hack-y with each passing year, at least for now you can still install it.Speaking of not checking... I've been having issues with my iMac so I'm doing a fresh install of OSX as restores from my time machine work for a couple of weeks and then freeze. I just looked at the minimum requirements and my Early iMac 2009 doesn't make the cutoff for MacOS Sierra. So it looks like I stop at 10.11.