I don’t see any reason why users couldn’t keep using this app after it’s discontinuation date. After all, what good does it do for users to not be able to decrypt their files after the fact?
I’m going to take a guess and say this app is probably tied into some DRM nonsense, which in turn probably requires downloading code from a server to run. This is a very short-sited way of writing apps, and all-too-frequently results in frustrating situations like this one. IMHO anything that can be done without relying on internet servers should be done without relying on internet servers. It’s the only way to make sure a random sever outage can’t cause an app to cease working.