It has been 11 years for iPhone and 13 iOS versions to be released. And usually Apple supports the new OS for the old iPhone for a very long time (about 3 to 5 years), however "the leaf is all gone" if the old iPhone is configured too low to continue to use well then Apple will stop providing new iOS versions completely.
Below is a chart that supports iOS on all iPhone lines over the years:
Is it a coincidence that Apple has "arranged" that the chart is very nice and logical. iPhone 5s may be the iOS supported model for the longest time from original iOS 7 to iOS 12.x.x today, totaling 6 iOS versions over 5 years.
However, Apple released iOS 13 after the recent WWDC event means that iPhone 5s and iPhone 6/6 Plus will only stop in the iOS version 12.x.x final. One of the main reasons is that these two iPhone models have only 1 GB of internal RAM, probably at the time of launch, the amount of RAM on the iPhone is not important to measure performance. But it took 4 – 5 years when the technology was more and more advanced, new iOS versions added smarter, more diverse experience, it was clear that 1 GB RAM could not meet (Note: 1GB RAM is only one part of the reason).
If you look back at the chart above, maybe next year iPhone SE and 6s / 6s Plus will be "on the road" soon when iOS is no longer supported. That happens, this chart will also look better in a ladder style however happy it is, hoping Apple will have some way to support OS for older iPhone models as long as possible.