You're describing the fragmentation problem--which is what we were discussing this afternoon--exactly. Some people won't use proprietary products. Some can't be bothered with IRC. Some of those will use certain proprietary products but not others because it means they have to use a client for one particular purpose.
A lot of people find email lists "old school."
So you end up with fragmented communication channels. Everyone is "happy" but you end up with a number of different sub-communities not talking to each other. It's worse than it's ever been IMO.
That's a real problem, but actually I wasn't describing it.
What I'm describing is how I found Slack difficult to work usefully with to a significant degree, compared with other platforms I've used.
Therefore, when evaluating its effect on community growth, it's not as simple as "use Slack nowadays to grow an open source community", because it acts as a significant repulsive barrier towards some of us (who are relevant to that community), even if it's attractive towards others.
It's not so much fragmentation as functionality; and those might be solved if there was better federation between systems to allow a broader range of tools.
The reason this is not just fragmentation is illustrated by my result with the Rust & Go communities: After trying out the Slack channels, and finding myself only dropping in once a month or so for a peek due to the tool being clunky, I lost the motivation to go looking for other fragmented sub-communities until I'd eventually find a "good" one. As a result I haven't gotten involved in any, despite the initial interest. I'm too busy with other things, and other communities. So I'll just passively consume the product now for a while, until reasons to put in the effort come up again. And I'm someone who is actively interested in the projects as a PLI person. Just an anecdote, but I think this repulsion effect is not great for open source community building.
But if they're not interested in using it for community communications, the fact that they already have an email address doesn't much matter if they won't use it for that purpose. They have a phone number too but they probably don't want to use that either.
There's no great solution. Ultimately you mostly have to meet people where they are or are willing to be. And that probably means that different groups are on siloed channels.
A lot of people find email lists "old school."
So you end up with fragmented communication channels. Everyone is "happy" but you end up with a number of different sub-communities not talking to each other. It's worse than it's ever been IMO.