This may work with a well functioning team and organization where everyone has basic literacy skills, but I suspect every big co is similar.
I may make the same grammar and orthography mistakes, but at least there is not an additional pronunciation challenge.
But I do consider myself fluent in English (even if far from perfect). I don’t know if that would still be true to someone not fluent in English
One of my language teachers said babies learn listening first, then speaking, then reading and finally writing, whereas adults it is the other way around: adults find it easiest to deal with writing because they control the context and it is in their own time, then reading because although there might be unexpected constructions, it is again in their own time. Then speaking for the same reason as writing, except timing is more real time, and finally listening is the hardest challenge, because it requires real time understanding of the context.
So I agree that I would expect written communication to be easier to deal with for many non natives of any language.
In my experience written communication worked a lot better when people were not at ease with the subject or the language, as you could do it asynchronously and they could edit themselves easily.
Meetings had an incredible amount of back and forth with ton of whiteboarding and people only reacting after we'd come to a conclusion that we rewrite in plain words. It's still a pain to do that in written form, but each side isn't stuck in meetings for 5 hours at least.
Otherwise, yes, in that case there's just no way around voice communication.
If they can't speak fluent English, then long-form messages are beside the point; they're beyond the reach of either Slack or face-to-face meetings. I've never worked for a firm in which a significant part of the staff were functionally illiterate, I'm glad to say.
> I suspect every big co is similar
That doesn't gel with my experince.