Downloading 32 bit iOS Apps for 32 bit iPhones after iOS 11 is released
While Apple is moving to 64-bit apps on the App Store, that doesn't mean they're 64-bit only apps. In fact any new apps published since 1 February 2015 has had to support 64-bit. This is also the case for any updates to existing apps (i.e. they also must be 64-bit compatible). 

In other words, any 32-bit only apps submitted to Apple will be rejected, but if that same app also supports 64-bit, then it'll be available for both 32-bit and 64-bit versions of iOS etc.

In summary, any app that:


is only 32-bit is on borrowed time
is only 64-bit is fine
that supports both 32-bit and 64-bit will be fine