Yes this most likely comes from Biome/KnowledgeC data which can persist for 30 days. This is not unique to signal but a bunch of apps.
iMessage, Instagram, Facebook, Discord are good examples as they rely on “iOS-level” notification hiding which does nothing in reality.
Signal, Telegram, and WhatsApp have options to sanitize the notification in-app which is the “real protection” against this.
Only way to wipe notification remnants is to factory reset the phone and NOT restore from a iCloud Backup. Your iCloud backup can reintroduce old forensic artifacts. If you have iCloud+ then using iCloud backups would be silly. Most of your important stuff is saved as synced data
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Photos (iCloud Photos)
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Messages (if Messages in iCloud enabled)
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Contacts, Notes, Calendars
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iCloud Drive files
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Keychain (passwords)
Extra Note: It is also important for everyone you communicate with to be just as educated as you or all of this is for nothing. The FBI used one persons phone (who deleted the app) to access the messages of other people. This serves as a reminder to not blindly use these secure platforms without understanding at least some digital forensics. People bash the cloud for being insecure yet are harvesting a gold mine of data on local devices that could be accessed depending on their security posture.