How to install iOS 14 on an iPhone 6?
I think the main difference between the iPhone 6 and iPhone 6s that stops iOS 14 supporting the iPhone 6 is probably the fact that the iPhone 6 has 1GB of system RAM (not storage), and the iPhone 6s has 2GB of system RAM.
There is not a lot that could be done to make an OS designed to run in at least 2GB of RAM, work on a system with only 1GB of RAM.
Even if you were able to persuade iOS 14 to install on an iPhone 6, it would run like a dog, if it would run at all.