“When you use Signal, your data is stored in encrypted form on your devices. The only information that is stored on the Signal servers for each account is the phone number you registered with, the date and time you joined the service, and the date you last logged on.”
Fun fact: on Android after not using an app for more than three months (the exact time can vary by manufacturer) the OS will remove permissions including notifications (suppressing the app’s ability to run in the background) to save battery. So the app will literally stop working.
For a seldom used app like Signal that’s hurt from a lack network effect, this triggers a death spiral outside of the privacy enthusiast community. When it had SMS support this guaranteed some usage, but now that’s gone.