The only reason for already published apps to stop working as expected is if they rely on any of the Telerik Platform Backend Services
(also known as Everlive) - for example, data storage, user management or push notifications. If your published apps use any of those services or make calls to the Everlive API, you need to migrate them to an alternative solution before the discontinuation. Otherwise, the apps might crash sporadically or exhibit unexpected behavior.
Apart from that, we are not aware of anything else that can lead to problems with your published applications.
I hope this is of any help.