I have a bit of a different take on this. Personally speaking, I've only been a Slack user for 2 years. Prior to that, I lived out of Skype for Business (now Teams[0]).

On one hand, I've had each of the issues he lists in his bullet points when dealing with Slack (though we only ever join customer's Slack channels), though I've adapted my approach and it's been minimal-to-non-issue, lately -- and I've got no customers asking for this at this very moment. The thing is -- I do not and have not had these problems on Teams.

I believe these sorts of problems aren't general problems that messaging applications introduce, but are problems Slack doesn't handle well. At first, my thinking was "everyone can have it, so it's like the old days of AIM, except people use this for business and a lot of businesses use it", so I went to "it's a public form of AIM that corporations are asking employees to use". None of that is particularly accurate, and it ignored the fact that every OCS - Teams installation that I've used had been federated with people I worked without outside of my organization. Slack didn't really "mainstream" this sort of thing with the people I worked with, so why do people on Slack behave differently?

The only reason I can guess is "Status". Microsoft always pushed this as a huge selling point to their product and I remember thinking "that's dumb" for the first several iterations, but right around the Lync time-frame, I realized that it created a behavior change in me. We used OCS for everything, including telephony, so when someone had a meeting blocked out in their calendar, or was using their phone, their status said "In a call" or "In a meeting" and was bright red. When you shared your screen, your status went do not disturbt[1]. Screensaver pops up and your status goes yellow "Away". You can also set these statuses, manually (which was rarely abused at the 3 shops I was at).

Another important factor was that chats were not persistent[2]. This changed with Teams, and the jury is still out as to how this impacts things, but I think it will make the specific bullet points he mentions worse. In general, when I would send someone a message on Teams, I'd not expect an immediate reply -- even if their status was green -- and if I didn't get a reply, I'd follow up with another method of communication. The idea was to avoid a phone call for something easy. Yes, this eliminates a lot of the "personal hands-on sort of interactivity" that a phone call grants, but it also eliminates 10 minutes of talking about spouses/families when you and I are in the middle of 3 other things and aren't terribly interested in keeping up with cultural norms.

Because status gave me enough information to answer the questions (1) "Are they likely to respond?" and (2) "Am I likely to interrupt them?". I did a lot of development for this platform and I had a ~15,000 person installation to get stats from -- it's interesting to see who had "status change alerts" setup for whom, when they'd be turned on/off, how often people would set their own statuses vs. using the one automatically set. It worked about as I described; replies fell off when statuses were not green, follow-up phone calls frequently followed unanswered chats -- all of this data was relatively easy to grab out of the massive set of SQL databases that powered the whole thing on-prem.

Contrast that with Slack. Unfortunately, at my new employer, we use Slack and are migrating to Teams, online, so I can't get stats like that any longer and my only visibility is myself and my immediate coworkers. There are, effectively, three/(four?) -- mostly worthless -- statuses, some of which behave in a manner that contradicts the features they provide:

(1) Zzzz - it's before or after work; useful when I need to reach out to folks in Seattle and I forget how to subtract by 3. Otherwise, as useful as a clock or my own two eyes looking at the desks nearby. And why warn me not to send this message? You're going to keep it there until they wake up and look, so how about just warn me that you're not going to notify them because they've snoozed alarms, but that the message will be waiting for them when they return?

(2) Offline - grey circle - they're not there or left, or maybe they went to screen-saver or their client as acting up. The thing is, I rely on status so minimally that I can't remember if there are two to cover Offline and Away. Both mean "don't bother" even though they shouldn't -- they're saving my history (if I'm paying, but even if I'm not ... for at least a few hours!)

(3) Online - green, solid, circle - they might answer me.

In addition to that, you can set an icon with a secret (/s) message. I say "secret" because nobody ever sees or reads it. I set mine last week to a "BIG RED STOP SIGN" with a message indicating that I was focused solely on a single project and in the middle of a crunch. I've been done with that for a few days; I received more messages about other projects I had to table last week than any other time working here and this message was so noticeable that I didn't even remember to turn it off until I looked at it just now for the purpose of this post.

So people message more. And they still follow up with phone calls and e-mail, just like they did before, but the messages are reaching their audience less frequently. When people at my last employer sent messages on Teams (Lync at the time), it was 85-90% targeted at "Available" and "In a meeting[3]". I'd bet it's the same way now, only "Available" covers a whole lot of time when nobody is available, causing more messages, less success and more frustration with the product

[0] To be accurate: I lived out of Office Communications Server (OCS), OCS R2, Lync, Skype for Business (and all of the CUs in between) -- I used to develop for these platforms so I was an early adopter.

[1] So intelligent -- never have someone send you a sarcastic or embarrassing comment while you're sharing your screen.

[2] Not, strictly, true and not true at all with Teams. If administrators enabled the capability, chats were stored in your e-mail box where they might as well have been deleted. Group Chat behaved exactly the opposite -- you had to install a massive client and all chat logs were kept forever with no way to delete them IIRC. Very few actually used this product. Teams behaves more like Slack but with all of the capabilities of Skype for Business.

[3] This was puzzling; we assume it was because the vast majority of meetings were conference calls after full adoption of OCS R2, so "In a meeting" meant "I have a meeting on my calendar that got cancelled" or "I have a very rare, in person meeting with only people in the office", otherwise it said "In a call". So really, it became "Extra Available" since you had time blocked out that freed up.