Plesk build constantly evolving software, which overtime can become obsolete, giving way to a new major version. Below one can find typical release lifecycle of any publicly available product.
Release / General availability - new product is offered publicly, it receives bugfixes, new features and updates on regular basis, technical support is offered through all available channels
Extended Support / End of maintenance - Product stops receiving new features and bugfixes. Critical fixes, security updates and technical support still provided.
End-of-life (EOL) - Product development is ceased, technical support is no longer offered