Apple iPhone 6s/Plus country availability map and LTE bands compatibility
Apple iPhone 6s and 6s Plus rollout schedule and country availability map
Of course, Apple is cautioning that it depends on individual carrier settings, saying that "unlocked models may support LTE networks outside the country of purchase when using a valid SIM from a supported carrier. Contact your carrier for more details," but if history is any indication, most of the times you can just pop a local LTE SIM card if you've bought your device unlocked, and enjoy the blazing download speeds that the local carrier in question offers you. Here are the countries and carriers each different iPhone 6s or 6s Plus model number supports.
Things that are NOT allowed: