Right, that's a legitimate security concern but the point I was trying to make is that the problem is not so much people using unapproved chat apps but people p

Right, that's a legitimate security concern but the point I was trying to make is that the problem is not so much people using unapproved chat apps but people putting sensitive information in chat apps at all (approved or not). These applications aren't generally designed to have the sorts of access controls required to manage sensitive information. If I post something sensitive in a public slack channel at my work (where we use Slack) then it is available to anyone in the company. I guess it is marginally worse to have it available to everyone in the company plus any former employee who wasn't removed from the slack team, but only marginally.