That's besides the point. Why block users from using it if they want to so? If it works without any extra maintenance, great. Which, apparently, it did.
Unless Slack has material amounts of support due to mobile users trying to use its web app, which I'm very skeptical about given that the app is a dumb wrapper around a web view anyway, there's no sensible reason to actively prevent those users from using it.
What likely happened instead is that someone at Slack is trying to growth hack app download numbers.
On the contrary. It is so important that it cannot be secondary to something that can be defined clearly. Hence my comment.
Edit for clarity: What you think is fine or not today may or may not be considered fine in the future. So there's an incentive to not set what is intolerance in stone on the basis that today's standards may not match those in the future.
Frankly, I don't know what a specific legal criteria might even look like. In fact, I'm not even sure I'd like someone smarter than me with legal street credentials to come up with one.
What I do know is that SCOTUS famously refused to define what porn is [0], and went instead with something to the effect of: I know it when I see it, and this is not it.
Personally I'm in agreement with Popper's view that societies should tolerate everything except intolerance. It draws a fine line between what's acceptable speech and what is not. And going by it, things like 8chan should get shut down.
The line of thought you put forward, by contrast, rubs me in a very wrong way. It was used to justify, depending on the period and country, not allowing people to vote on the basis that they didn't have enough revenue, didn't own enough land, couldn't read and write well enough, etc. Allowing to disenfranchise voters on some arbitrary sophistication basis can and, if history is anything to go by, unfortunately will get abused. It breaks down to: who decides what's sophisticated enough?
Popper's tolerance criteria, by contrast, seems clearcut in a you know it when you see it kind of way.
I presume you haven't submitted candidates to a fizzbuzz test. Try it. The test is well known and out there in the wild. It's almost part of programmer culture. Yet it is absolutely shocking how many candidates that apply to jobs that involve programming will fail that simple test or some minor variation of it. Almost as shockingly, they will also fail to spot their bug after producing a solution that doesn't work as expected.
Is Linux that popular with youngsters after they've left university? When I think about laptop Linux users it's the archetypical greasy bearded GNU'er [1] that springs to mind...
Unless Slack has material amounts of support due to mobile users trying to use its web app, which I'm very skeptical about given that the app is a dumb wrapper around a web view anyway, there's no sensible reason to actively prevent those users from using it.
What likely happened instead is that someone at Slack is trying to growth hack app download numbers.