If Slack’s looking for features to add, my vote would be for personal “annotation” messages in threads, similar to what Hey mail has. Many a time I receive a support request as a Slack thread, and it’d be nice to add notes such as customer IDs as a message that only I can see.

Here’s a mockup:

A concept mock-up of a Slack thread showing interaction between a support person and the author, with the private note message highlighted in yellow with the disclosure 'only visible to you'

Edit: I knew I talked about this before. And looking through On This Day, I found this post, where I professed my wish for FastMail to add the same feature. Probably a good hint that such a feature should be table steaks for any system involving other people.