According to a report from France, owners of the iPhone 6 (2014), iPhone 6 Plus (2014), and iPhone SE (2016) will not be receiving the iOS 13 update, when it will be ready for prime-time. iOS 12, at the time of its release, went back compatibility-wise all the way to the iPhone 5s (2013) and iPhone SE, and it looks like with iOS 13, the list will only go back as far as the iPhone 6s and iPhone 6s Plus.
Such a move makes sense, as one year goes by between major iOS releases, and new iPhone models are being released. With the 2019 iPhones, Apple will be adding at least three models to its roster, pushing out older models from the compatibility list.
While this is just a rumor at the moment, so treat it accordingly, it not only makes sense for Apple to remove older devices from its iOS compatibility list, but it is something the company has been historically doing.