It's unlikely Apple would drop a capable device with an A9 and 2 GB RAM, one that was being sold as recently as 2018, or three generations of devices at once (5s, 6, 6s). But then again, I thought something similar about 2011 Macs still being capable prior to Mojave's announcement, so outside of Apple who knows.
[doublepost=1553957077][/doublepost]If the question is about the iPhone 6 (not 6s), then I actually think it's likely the 6 won't be supported due to the 1 GB RAM. The 1 GB was a big reason I didn't get one even in 2014, the performance improvements were so minor that it was almost just a 5s with a larger screen. The 6s brought more major improvements and I was glad to have waited.
Under this logic, the iPhone 6s / iPhone 6s Plus, iPhone SE, iPad Air 2 and iPad mini 4 are safe with 2 GB. iPhone 5s, iPhone 6 / iPhone 6 Plus, iPad Air (original) and iPad mini 3 are not.