If you use Signal, you actually have an advantage here, now that you know about this vulnerability. Signal has a setting that blocks the content of messages from appearing in their notifications. That way, even if someone accesses your alerts, all they'll see is you received a Signal message—not who sent it or what it contains.

To turn it on, open Signal, tap your profile in the top-left corner, then hit "Settings." Under Notification Content, choose "No Name or Content" to block all data to the alert. You can compromise here and choose "Name Only" if you want to know who a message is from before you open it—just remember, an intruder may also see you received a message from that person if they scrape your iPhone's notifications.