Great blog post on Slack's "bait and switch".
https://opkode.com/blog/slacks-bait-and-switch/
"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."