Even if your iCloud storage is full, the existing data on iCloud won't be automatically deleted. However, you won't be able to store new data or access some features like backups until you free up space.
Here's an important point: If you're storing Notes only on your device ("On My iPhone" folder), purchasing additional iCloud storage won't affect those notes. They'll remain on your device and won't be uploaded to iCloud.
Where were the Notes? The pic below is self-explanatory.

If they were stored on Gmail, Yahoo, or Outlook etc. then please add the account and enable Notes under them. See the pic below which is self-explanatory
