Two solutions...
For storing extra info with a password; for example, the subscription key for the products associated with the login. In the new Passwords app you can edit a password entry--analogous to editing a contact. Locate the item and "edit" it. Then put info in the Notes field. https://www.idownloadblog.com/2022/04/29/how-to-add-notes-to-icloud-keychain-passwords/
For saving secure info that's not associated with a specific login. Create a Notes entry and lock it with a password. In the previous Keychain Access app you could have a password on a complete keychain. The same password protected all notes in the keychain. In Notes you could use the same password for each note in a group of secure notes. FYI, I chose to put these notes in iCloud so I could access them across my devices. To help find them I created a Notes folder for my "secure" notes.
Now to figure out how to export secure notes from Keychain Access. May be easy, just haven't looked.