Which method is not available for interoperability between Google Workspace and legacy platforms?

The method not available for interoperability between Google Workspace and legacy platforms is federation for XMPP. Federation for XMPP (Extensible Messaging and Presence Protocol) is not supported because Google Workspace does not allow for the integration of XMPP services between organizations in the same way those services could be utilized in some legacy systems. While Google Workspace offers capabilities to communicate and integrate with other applications, XMPP federation is specifically limited and not an available option.

In contrast, IMAP protocols allow users to access their emails from various email clients and are essential for connecting to email accounts hosted on different platforms. Similarly, REST APIs are very much a part of Google Workspace's integration capabilities, allowing developers to create applications that can interact with Google services. ActiveSync is a protocol supported by Google Workspace that enables synchronization of emails, contacts, and calendar events across devices.

Each of these methods serves a specific purpose in enhancing the compatibility and functionality of Google Workspace with existing systems, except for federation for XMPP, which remains unsupported.