If I'm not mistaken, pull ratio changed in 2015. Pre-2015 11 speed rear derailleurs should work with pre-2015 10s shifters.
But even pre-2015 med/long cage derailleurs are rare or too expensive. You could try a roadlink, but you'll find the problem with chain capacity.
I'm not sure if "new" veloce 10sp derailleur works with 10s pre-2015 shifters. Anyone?