All new apps submitted to Mac App store must support 64-bit architecture and the existing ones have to be updated to 64-bit by June 2018.  The reason behind this is, Apple has made clear that macOS High Sierra will be the last OS to support 32-bit architecture. As of 31st January'18, new applications submitted to mac app store should support 64-bit architecture apps in Mac App Store and those existing have to updated to 64-bit  and support by June'18. 

Apple's reminder: 

"To prepare a future version of MacOS where 32-bit software will no longer work without compromise, starting with MacOS High Sierra 10.13.4. The user will also be notified when launching an application that relies on  32-bit software. The alert will appear only once per application."  The idea behind this is to create awareness to users and ask them to update the apps to latest versions.