From several months of experience, I believe that my initial answer (assumption) is incorrect. If the application has expired in iTunes Connect Beta Testing. It is very soon (one or two days after the expiration date) stop working with the installed device.
Original answer:
Short answer: when the testing period ends, testers will no longer be able to accept invitations and install assemblies. Testers that already have assemblies installed will not be affected.
In this document
https://developer.apple.com/library/ios/documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/Chapters/BetaTestingTheApp.html#//apple_ref/doc/uid/TP40011225-CH35-SW2
Apple says:
"After a 30-day test period, the build status changes to Expired."
βTo continue testing after a 30-day period, download another assembly. Internal testers will automatically receive an update notification when a new assembly is available. To distribute the new assembly to external testers, you must send it to the Beta application. as it is approved, you can send an update message to external testers by clicking on "Send Invitations" from the External Testers column in the Builds, as shown above. "
Apple does not clearly describe the behavior on the tester side when the build expires.
But , when I try to disable TestFlight test testing in the application, the prompt informs :
"Are you sure you want to stop testing? Testers will no longer be able to accept invitations and install assemblies. Testers that already have assemblies will not be affected."
So, I think the behavior on the tester side when the build time has expired will be the same as this when testing is turned off by disabling "TestFlight Beta Testing".
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