iPhone 6 and Older
Some iPhone 6 models will be able to get iOS 13, but not all of them. The dividing line is the “s” models. The iPhone 6s and iPhone 6s Plus will be able to download iOS 13 without a problem (although they won’t be compatible with new iOS updates forever). The iPhone 6 and iPhone 6 Plus will not be able to use iOS 13.
This is also true for the iPhone 5s and all older iPhones – although admittedly not many of these are still in current use.
If you’re having trouble visualizing the cutoff date, as a general rule iPhones sold before 2015 will not be compatible with iOS 13. The 6s models were released in the fall of 2015.