Which 27" iMac can be used as external display on a MaBook Pro Mid 2015?
From Apple KB: Use your iMac as a display


  Target Display Mode isn’t supported on iMac models with Retina display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.


That's it, just those iMacs, so Mid 2014 would be the latest.

After comments: It seems you additionally need to be on High Sierra or lower, support has been dropped in recent OSes - From Apple KB: Use your iMac as a display with target display mode


  Make sure that your iMac is using macOS High Sierra 10.13.6 or earlier. You can't use target display mode with later versions of macOS, or with Boot Camp and Windows.