Prior to 2015, LinkedIn had an email-like messaging system, where each message received was its own distinct item in the inbox.
There was a new initiative to change this to be threaded, more like a chat system. As part of my intern project, I worked on writing a new web application that would support this. This was also one of the first experiments of Ember.js at LinkedIn.
This project was initially written in Backbone.js, but halfway through, we were asked to experiment with Ember.js, which LinkedIn had already begun playing/building tooling around. We built a very rough MVP with Ember 1.0 (pre-2.0).
The product finally launched in September 2019. You can check out the LinkedIn blog post here. The final product actually ended up being a completely new app with Backbone.Marionette.