Found out the hard way that the old versions do stop working. You don't even get message notifications if your app is out of date.
That is a fine decision to make for a security-minded app, but signal has always presented themselves as a full alternative to SMS and other messaging systems where availability is prioritized over confidentiality and integrity. It should really be made more clear so that users are making an informed decision. They could also do wonders for the user experience by having the app inform the user of the problem and how to remedy it.