I think if we enabled outgoing webhooks into private groups ideally there would be an easy way for users to see what's been added.
E.g. an indicator in the member's list of a private group that indicates what integrations (e.g. outgoing webhooks, and what keywords they're listening for) are "members" of the channel, the same way end users would get an indication that a bot was part of the channel.
Our design principle is "fast, obvious, forgiving" and having outgoing webhooks in a private group with not indication to group members seems non-obvious that conversations could be monitored (versus public channels, where the expectation is that conversations are more open).
I think eventually we can add this, it just needs the right UI.