Your Slack login details are on GitHub
thenextweb.com
thenextweb.com
This isn't a slack specific issue, and it seems silly that its getting the attention that it is, just because of the word "slack"
The only reason anyone includes slack in this is because its a shiny new thing that is very popular right now (for good reason).
This is just another instance of the 'people committing credentials to public repositories' problem that has been around since before github even existed.
While I fully support efforts to mitigate this (github could and should track when people commit things like credentials, and warn them in some way) and they should facilitate with companies like slack to push to slack (so slack can warn/disable) lists of tokens this does not need to be some sensationalist thing and does not indicate any kind of vulnerability in either slack or github.