We saw it with Google, who built Gtalk on XMPP and even federated with other XMPP servers, only to later stop federation and XMPP support in favour of trying to herd the digital cattle into the Google+ enclosure.
Facebook, who also built their chat app on XMPP at first allowed 3rd party XMPP clients to connect and then later dropped interoperability.
Twitter, although not using or supporting XMPP, had a vibrant 3rd party client ecosystem which they killed off once they felt big enough.
Slack, like so many others before them, pretend to care about interoperability, opening up just so slightly, so that they can lure in people with the promise of “openness”, before eventually closing the gate once they’ve achieved sufficient size and lock-in.
On Federation:
The reason email is federated, is because it was developed before surveillance capitalism was a thing and because it was established and entrenched long before these companies came around.
There’s a reason why your email address is still the de facto way to sign up for any service on the web (sometimes with one or two degrees of separation), and it’s because of federation.
XMPP is designed to allow federation. Think about that. Instead of having to sign up to various different chat providers, all which try to lock you in and monetize your conversations, you could instead have one chat account, and use that to chat with anybody else, regardless of which chat provider they are using.
Alas, that’s the dream, but because XMPP came much later to the scene, it didn’t develop the critical mass as email has, and here we are. With dozens of chat apps, all non-interoperable and closed off.
This is the most reasonable response I have seen so far about the Slack vs XMPP/IRC debate over the past few days.
Related: GTalk