Nobody here knows but I’m willing to guess the A7 chips (iPhone 5s, iPad Air 1) would be next. The A8 chip models aren’t really losing their step in iOS 12 so it wouldn’t really make sense for them to cut those devices in iOS 13.
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.
A factory in India was built to manufacture the SE and 6S two years ago. It's hard to believe they'd can those models after 2-1/2 years, being that they need a less costly iPhone presence in India and there's no current SE size replacement.