Back in June when Apple first launched the iOS 10 beta, we reported that the OS would now give you a warning if you attempted to launch a 32-bit app, saying that it could affect system performance. There's a new warning in the iOS 10.3 beta, however, that now says that the app won't work in future versions of the OS.
Luckily, almost all of the apps that you use are likely running 64-bit versions, so you should have little to worry about. In fact, if you haven't seen one of the above-mentioned warnings, you'll be good to go.
The future version of iOS that's being referred to here is likely iOS 11, rather than iOS 10.4 or even a generally available 10.3. It also says something about the devices that Apple plans to support in its upcoming mobile OS, as the only 32-bit devices left are the iPhone 5, 5c, and fourth generation iPad, which run the A6 and A6X chips, respectively.
Indeed, this would seem to imply that those three devices will be dropped from support this September when the next major update to iOS is released, although we'll find out for sure when Apple announces it at WWDC. Of course, in order to eliminate all 32-bit app support, it would be necessary to eliminate 32-bit devices.