Blizz recently confirmed that the drop rates for Greater Rift Keystones scale with Torment difficulty. The drop rate is unknown, but it’s believed to be close to the Horadric Cache Legendary drop rate scaling, but T6 does not mean a 100% chance to get a Keystone of Trials — it’s closer to 90-95%.
Quote from Grimiku
You’ll need to be in Torment difficulty in order to have a chance to find a Greater Rift Trial Key. Each level of difficulty beyond Torment I will give you a greater chance of finding a key, too. (Source)