Erase signatures from the Mac OS Catalina V10.15 Preview app forever
Signatures are stored in a property list file at the following path:
~/Library/Containers/com.apple.Preview/Data/Library/Preferences/com.apple.Preview.signatures.plist

You can remove this file and empty your trash to be sure they're gone. Any security/assurance you need beyond this will require advanced techniques or a complete wipe of your hard drive.