“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.”
This only ever worked on Android so while helpful, it wasn’t a panacea by any means as it didn’t actually reach most people. Last time I had some numbers, the downloads across the App Store and Play Store were very similar so it probably ever reached about half the users. That’s if we assume everyone on Android used it, which wasn’t the case. 🤷