Plus (legacy) account status refers to those that paid for Day One between February 4, 2016 and June 27, 2017. It is a per-platform status based on your purchase history. For example, if you paid for Day One on iOS only, you will have Plus on iOS only. For a list of features included in the Plus status, see the Pricing Guide.
The Plus account status is determined by the Apple ID that was originally used to buy Day One. The Apple ID is essentially the license for Plus. This is not necessarily tied to your Day One account, but the Apple ID used in the device Settings/System Preferences.
If you should have Plus on a device but see Basic instead, you can follow these steps to resolve this issue:
iOS
Please use Day One > Settings > Advanced > Restore Purchases to restore functionality.

If the above steps don’t work, please try this with your devices:
Try restarting your device and seeing if “Plus” status appears in Settings.
If that doesn’t work, try an uninstall/reinstall. Please verify your data is synced at dayone.me before reinstalling.
macOS
Go to Day One > Preferences > Support > Restore Purchases

If the above steps don’t work, please try this with your devices:
Try restarting your device and seeing if “Plus” status appears in Settings.
Lastly, you may need to sign in to the App Store on the device with the Apple ID of the original purchaser and then restore the purchase in the app.
After trying the above, if you still don’t have Plus, contact support for additional help. Sorry for the tremendous inconvenience this has caused, and thank you for your patience.