In the US, with a US-originated iPhone 6s (Verizon), I also encountered this battery issue (randomly shutting down between 17-45% without warning) and Apple replaced it under their recall program (serial number eligible). Apparently, during the layering/rolling of lithium electrolyte, electrodes and cladding included in early examples of iPhone 6s'es, there was too much air (impurities) introduced into the resulting battery, shortening its life and making it behave unpredictably (even more nonlinearly) at lower voltages (low charge). Instead, I assume from the root-cause analysis provided, Li-ion batteries need to be assembled in a vacuum (ideally), under a noble gas (Argon) or inert gas (nitrogen only).
Maybe someone with more information than the article about how the issue was originally uncovered and how regulators factor into it and Apple's response. For example, does Apple not offer the recall uniformly or were they forced/suggested to make additional remedies (beyond those mentioned in the article)?