Signal no longer working on iPhone 4S
Solution 1:
According to Signal support:
Confirm that Signal is supported by your iOS. Signal iOS has a minimum iOS requirement of 11.1
Highest version iPhone 4S supports is iOS 9.3.5
Solution 2:
I contacted Signal Support, per the comments, and the reply was:
Signal supports older OS's for as long as possible unless there is a technical limitation. This is not a decision we took lightly. There are new features we want to add to Signal that will not work on older iOS's. Signal requires iOS 11.2 and later.