When I ran my own consultancy, I went on holidays for a month once, leaving the company under the management of a long time employee.

I came back to find an employee of one of my customers sitting at a desk in our office. Turfed him out. Had to "fire the customer". Lost about 30,000 euros of work time and unpaid invoices.

In my view joining someone's Slack Channel would be no different than that little piece of "management kidnap". Everyone in the office had completely turned their attention on that single customer's problems.