Edit: My bad I had a terrible typo in my original post where it said 'employee hosted' and not 'employer' as I intended.
I think the folks working towards that stuff would be much better served to have their own slack disconnected from the employer's systems.
Whether that means communication that happen in meeting rooms, in break rooms, in cafeterias, or over e-mail, Slack rooms, or other employer-hosted message boards.
It's important to have union communication spaces outside the workplace too -- whether at union headquarters, Slack rooms, etc. -- but nobody's even going to find out about those without discussion and recruiting happening on-site.
If employees are allowed to approach each other about organizing in a cafeteria or break room or by the water cooler (which is critically important), then Slack is no different -- the "social" Slack channels are just the digital equivalent.
I can reach all people in a Slack channel by spending a few seconds typing something. Reaching any large number of people through chance encounters in an office on break time requires perhaps hours of work spread out over days or weeks. And you'll never reach employees who work in another location if you can only physically talk to the ones who work in your office.
Slack is, the whole setup is a mess.
As someone else has posted, once you let a little non-job related material in ("cute doggo pics!!!") you're almost always allowing labor discussions in as well. A strong pro-labor NLRB would (and possibly will) be all over this action.