Uber currently uses Mattermost but will most likely move to Slack.
During my 2 years at Amazon, the SFO based Prime Now team used Mattermost but we were eventually forced to move to Chime a pitiful example of a chat client.
Mattermost is already certified on Aurora, we work with S3, it's available in AWS Marketplace--if there's a dev team at Amazon interested in an open source alternative to Slack that runs natively on AWS infra--potentially connects with Chime for voice/video/meetings/screenshare it's potentially a healthy thing for everyone.