Andrew Cunningham, Ars Technica:

Beta builds of iOS 10.3, the first of which was issued last week, generate warning messages when you try to run older 32-bit apps. The message, originally discovered by PSPDFKit CEO and app developer Peter Steinberger, warns that the apps “will not work with future versions of iOS” and that the app must be updated by its developer in order to continue running. The apps still run in iOS 10.3, but it seems likely that iOS 11 will drop support for them entirely.

Apple planted the seeds for this a few years ago when they mandated that all new apps be 64-bit compatible. Then, last summer, warning alerts appeared when 32-bit apps were run, reminding the user that using the app “may affect overall system performance”. So this is simply the next step in the progression.