Every open source project that’s reasonable enough not to hide all their communications in proprietary silos like Slack and Discord.
Seeing that both slack and discord save logs from the beginning (which irc doesn’t), isn’t communication less hidden if anything? It’s far easier for someone new to play catch up.
There’s a reason large open source projects moved to these (React, Rust, etc).
That is, you can ping people who are offline, or with a flaky network connection (e.g. mobile, where you can't keep a consistent TCP stream open), and they can still see your message and a bit of context when they get back.
By the way, I find IRC/Slack good for real-time support. By nature of being real-time support, some questions get repeated over and over again. The repeition is typically mitigated by adding trivia bots to the channel that can provide answers to frequently asked questions.
The best solution I have seen for archived solutions and to avoid repetition are Stack Overflow (encourage users to post questions on SO with relevant tag) and mailing list with a mailing list archive.
In Slack the logs are also not searchable if you above a certain threshold (10k lines or something) if you don’t pay. Slack is prohibitively expensive if you have a lot of users and not really made for such a non-company use case.
Not everyone uses IRC (but it has archives). Email (and its archives) are a thing, too.
Given you can read all the development of the Linux kernel since almost the beginning (and they don't use chat IIRC), I don't think you are correct.
> There’s a reason large open source projects moved to these (React, Rust, etc).
No, there is no reason. They are newer projects with younger people and they just picked something more modern, based on a web browser.
If there was truly a reason, the kernel and other projects would have moved, too.
You can argue, of course, the pros and cons of each alternative.
I believe it was sort of catalysed by Mozilla shutting down its IRC servers, but presumably it was already an issue being discussed, and that was just the final nail.