No Rust did actually move (its 'official') channel off IRC.
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.
And since that happened, I've stopped engaging with the community at large. There are people who disagree, but I personally do not want to waste stupid amounts of memory to do ephemeral messaging. And their gatekeeping bot is stupid.
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.