These documents are new in the last few days:
https://slack.com/blog/news/how-we-built-slack-ai-to-be-secu...
https://slack.com/intl/en-gb/blog/news/how-slack-protects-yo...
I think these updates are really good - Slack's previous messaging around this (especially the way they unclearly conflated older machine learning models with new policies for generative AI) was confusing and it wasn't surprising it caused a widespread panic.
It's now very clear what Slack were trying to communicate: they have older ML models for features like channel recommendations which work how you would expect such models to work. They have a separate "Slack AI" addon uou can buy that adds RAG features powered by a foundation model that is never further trained on user data.
I expect nobody will care. Once someone has decided that a company might "train AI" on private data you've already lost that person's trust. It's not clear to me if any company has figured out how they can overcome one of these AI training panics at this point.
I wrote a bit about this back in December when it happened to Dropbox - there is an AI trust crisis at the moment: https://simonwillison.net/2023/Dec/14/ai-trust-crisis/