David V. Kocher, the lead developer, has publicly stated multiple times on the Cyberduck mailing list and GitHub issues:
Cyberduck is a popular, user-friendly file transfer client for macOS and Windows that supports FTP, SFTP, WebDAV, Amazon S3, Google Cloud Storage, Azure, and more. Many users appreciate its clean interface, integration with system tools, and support for cloud storage providers. One topic that frequently comes up in forums and comments is the “Cyberduck registration key.” Here’s a concise, useful blog-style post explaining what that means, what to expect, and how to proceed. Cyberduck Registration Key
Free/Donationware; registration key only removes prompts. David V