I don't think any iOS devices with the A7 or original 1GB A8 will be getting iOS 13. Seeing as how Apple just released a 'new' A10-based iPod touch, and the only devices they still sell with an A8 is the 4th Gen Apple TV and HomePod - both of which run a lighter version of iOS - I think the iPhone 6 will be dropped.
I believe this will be caused by a lack of RAM. The original A8 found in the iPhone 6 and iPod touch 5th Gen did not have enough RAM or power to be future-proof, and I can speak from experience with a launch iPhone 6 Plus that it was slower than it should've been for a brand new phone. I'm thinking the iPad mini 4th Gen and iPad Air 2 may be saved from being dropped, seeing as how they have 2GB of RAM, and the Air 2 has 3 CPU cores. I have family members who still use iPad Air 2s on the latest version of iOS and it does not feel like a 5 year old tablet.
My prediction of supported devices on iOS 13:
iPhone 6s/6s Plus (partial)
iPhone SE (partial)
iPhone 7/7 Plus
iPhone 8/8 Plus
iPhone X
iPhone Xs/Xs Max
iPad mini 4th Gen (maybe partial)
iPad Air 2 (partial)
iPad Pro (all models)
iPad 5th Gen (2017)
iPad 6th Gen (2018)
iPad Air 3 (2019)
iPod Touch (7th Gen)