Apple just announced the iPhone SE, a 4-inch phone with the guts of an iPhone 6S that looks a lot like the 5S. So, naturally, Apple has apparently discontinued the iPhone 5S. The 5S has been replaced by the SE on the iPhone hub of the Apple website, and the old URL that brought you do the 5S order page simply redirects you back to the main iPhone page.
The news comes as no real surprise; Apple discontinued the iPhone 5 when it announced the iPhone 5c, and then it discontinued the 5c last fall. With a much more powerful iPhone SE being made available by the end of this month, Apple apparently sees no need to keep selling the underpowered 5S; a shame, because as nice as it is to see Apple embracing the sub-$400 phone wars, it'd be even nicer to see a truly cheap 5S kick around on store shelves for a while. (Though, if you're quick, you can probably still catch some of the remaining stock before the 5S is really gone.)