Maybe there could be an entry in the notes’ metadata, saying which notebook it was from? The ‘restore’ could then move it back there (and remove the info from metadata).
Also not sure about the “regular notebook” bit - one would preferably want special handling, like having the notes for read-only access. (It would only lead to confusion - I’m editing a file in this notebook, then suddenly after 30 days my note is gone. )
Also, why not have it synced? If I’ve effed something up on mobile, and realize it while on the desktop, I would really like the option to restore it from there.
And while I’d like a “delete everything older than 30 days” function, I would never want it to run automatically. (Sometimes I’m sorting my notes “when I have time”, which may take a looong while, and I like to know they stay in the trash bin until I’m sure the process is done and I no longer need them.)