They use your phone number when you signup. They then cross reference that phone number with peoples contacts who have already downloaded the mobile app and voila they recommend people who have your phone number stored on their phones. I know this because I just got a new phone and it only recommended people whom I gave my phone number to.
YouTube does not support this kind of thing, you need to specify which email addresses exactly should have access.
As a workaround, you can record a screen capture of you performing the steps to reproduce yourself and upload those as a public video.
As of May 2019 Slack's privacy FAQ states that on a free plan, owners can request an export of all data, including private messages and channels. However, they must "provide (a) valid legal process, (b) consent of members, or (c) a requirement or right under applicable laws."