Is Nucleo a subscription?
The Nucleo application is a free icon organizer for Mac and Windows. You can download it for free and use it without purchasing the Nucleo icons.
The Nucleo icon library is a premium set of icons optimized for the Nucleo app. The purchase of the Nucleo icons is not a subscription, but a one-time payment.
After the purchase of the icons, you can unlock them inside the app by logging in with your account credentials.