iOS 14 works great on 6s (what I currently have) and I assume the 7 and 8 which all have 2GBs. iOS is so well designed and optimized that it doesn't require huge amounts of RAM like other devices. Apple did blow it big time with the iPhone 6 and its 1GB of RAM but all devices with 2GB or more have done well for years. I can imagine 4GB and 6GB with the newest chip will mean iOS runs flawlessly with the 12 series of iPhones.
It would be nice to see these tech specs, but for whatever reason, Apple has never disclosed that. My guess is to avoid apples and oranges comparisons with android devices which don't have the optimization as iOS devices.