Old notes auto-delete on a schedule you choose, and slotted notes never expire.
Notes that have not been modified for some time will auto-delete. Slotted notes never expire.
To clear old notes in one sweep, use "Delete all notes not modified since" in Settings > Notes — it confirms first and moves the notes to The Void, so they stay recoverable.
Settings
Auto-delete unmodified notes after — Today | 1 week | 1 month | 1 year | Never
Delete all notes not modified since — date + Delete… button