In What we’ve learned from our users about designing for accessibility Andrew Gosine describes how their team lived out one of Slack’s primary design principles, which is to design with users, not just for them:

In another proposed update, we tried to get clever about where we placed a user’s focus when they opened a thread. If there was an unread message, we’d drop them into that first message in the thread. If there were no unread messages, we’d move focus to the message input. We believed this would increase efficiency for screen-reader users. Our feedback group reacted strongly to this. We’d unintentionally deteriorated the reliability of knowing exactly where you would be when you open a thread, and, as a result, we broke the way-finding our users relied on in Slack. Thanks to our group, we reverted that change.

This is a great read with lots of examples from the project.