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.