Google Talk did something that now feels almost exotic: it let people on other compatible messaging systems talk directly to Google users.
The service used XMPP, an open messaging standard. Google’s own developer documentation described Google Talk as fully interoperable with other services supporting server-to-server XMPP federation.
https://support.google.com/code/answer/56278
That meant the network boundary was softer than the company boundary.
Federation did not require Google’s permission
Google’s documentation said outside service providers did not need special approval to federate with Google Talk. If their servers supported the XMPP standard, their users could communicate with Google Talk users and vice versa.
https://support.google.com/code/answer/55704
Another Google help page listed Earthlink, Gizmo Project, universities, corporations and individual XMPP servers among the kinds of systems that could participate.
https://support.google.com/code/answer/55703
That is genuine interoperability. A person could choose a different provider or client without necessarily abandoning everyone on Google Talk.
Then the product direction changed.
Hangouts replaced Talk without XMPP federation
In May 2013, Google announced Hangouts as the replacement for Google Talk. Google’s old developer documentation now carries the same blunt notice on multiple pages:
Hangouts does not support XMPP.
https://support.google.com/code/answer/55693
The messaging service still worked. Google users could still talk to Google users. What disappeared was the open bridge that had allowed independently operated XMPP systems to participate in the same conversation network.
That is a different kind of product change from removing a button.
A button can be relearned.
A protocol boundary decides who can connect at all.
Adoption makes lost interoperability expensive
Once enough contacts, organizations and conversations accumulate inside one provider, losing federation creates a practical choice for outside users: move closer to the dominant service or accept reduced reach.
Google had legitimate product reasons to build Hangouts differently. Messaging had expanded beyond simple text and presence into richer media, mobile synchronization and tightly integrated Google services. Open standards can also constrain how quickly proprietary features evolve.
But that tradeoff belongs in the record.
The original system advertised openness as a feature. The replacement system kept the audience while dropping the protocol that made cross-provider communication possible.
Interoperability is most valuable before a platform wins.
It is also easiest to remove after everyone has already arrived.