So Google was really embracing the federation. How cool was that? It meant that, suddenly, every single Gmail user became an XMPP user. This could only be good for XMPP, right? I was ecstatic.

.

First of all, despites collaborating to develop the XMPP standard, Google was doing its own closed implementation that nobody could review. It turns out they were not always respecting the protocol they were developing.

Federation was sometimes broken: for hours or days, there would not be communications possible between Google and regular XMPP servers. The XMPP community became watchers and debuggers of Google’s servers, posting irregularities and downtime

And because there were far more Google talk users than "true XMPP" users, there was little room for "not caring about Google talk users"

.

In 2013, Google realised that most XMPP interactions were between Google Talk users anyway. They didn’t care about respecting a protocol they were not 100% in control. So they pulled the plug and announced they would not be federated anymore.

As expected, no Google user bated an eye. In fact, none of them realised. At worst, some of their contacts became offline. That was all. But for the XMPP federation, it was like the majority of users suddenly disappeared. Even XMPP die hard fanatics, like your servitor, had to create Google accounts to keep contact with friends. Remember: for them, we were simply offline. It was our fault.

https://ploum.net/2023-06-23-how-to-kill-decentralised-networks.html