Settings > Notifications > Notification Content > Show: "Name Only" or "No Name or Content"
I've had this enabled to prevent sensitive messages from appearing in full whilst showing someone something on my phone, but I guess this is an added benefit as well.
Fwiw, in my Signal app on Android this setting is in
Settings > Notifications > Messages > Show
Disable Apple Intelligence summaries for sensitive app notifications too.
I guess enabling Lockdown mode might avoid this particular issue too, together with a bunch of other stuff?
Just to clarify, this is within the Signal app settings—not the OS (iOS or Android) system settings.
Critical distinction, as merely changing OS notification settings will simply prevent notification content from being displayed on-screen.