Where is iCloud Drive Sync?
It is not recommended to use iCloud Drive to sync your notes, because NotePlan can't control when the sync should happen. The result is that you need to open the Finder/Files app and navigate to iCloud Drive to encourage the sync sometimes or it won't sync it for some time. CloudKit, on the other hand, enables almost real-time sync and saves your notes still as plain files to a folder on your device.
If you heavily rely on it and want to use it despite all warnings, you can still unhide the button in the preferences by clicking on the following link (on iOS and macOS separately, this is not getting synced across platforms; reopen the preferences after this): noteplan://x-callback-url/showiCloudDrive