It never struck me as good company policy to have all leave requests in a public channel visible to all company employees.

I think that this solution came about because the company had a very strong dislike of email for some reason. All support was done using Intercom.io, rather than email and all company communications were done in Slack. I don't think I sent a single email in the 2 years I worked there.

Being able to create a direct message with a topic would've been the ideal solution, which is literally what email is.