I actually spent a bit of time searching for a letter she sent with an ultimatum to her management. I've seen this referenced multiple places as "the email that got a Google researcher fired": https://www.platformer.news/p/the-withering-email-that-got-a...
But that doesn't seem right. That looks more like a different type of letter, sent to a wider audience than just Timnit to her manager(s).
I've seen referenced that there was a timeframe for when she was going to quit that she mentioned. I see no such reference in that letter.
Maybe there's a real copy of the letter floating around. My guess is that people pretend that they've seen "the letter" but only read this other one I referenced above.
I'm happy to be proved wrong, though. I have no dog in this fight except to understand the truth of the situation. If you've seen a different letter than shows her ultimatum and time frame, please share.