Realistically, discussions about unions and organization are always going to need to happen at least partially using "employer hosted" spaces.
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 don't think it's at all reasonable to say that a Slack channel is an "equivalent" to spreading information by ad-hoc meetings in the cafeteria/break room/near the water cooler.
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.
I think there is a difference in the slack channel being hosted on Apple's Slack compared to an employee telling another about their independent Slack instance & channel over an existing Slack channel and/or DM.
I think you should look some of this stuff up. Your expectations aren't really in line with the law.
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.
Right, this is why lawyers are renowned as the fun police. The correct action from the perspective of the interests of the shareholders would be to ban the dog chat and the dad jokes.
Only if management and the shareholders are ignorant and short-sighted. Discouraging union activity with a stick is not a long term win. Maybe with a carrot, or maybe embrace it.
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.