Draft message and schedule send
Workplace by Facebook as those features. You can save as draft and easily find your draft later. Also have an option to schedule when your message is sent (great for announcement, recurring messages or to not bother staff outside the business hours)
Charles Fordin commented
A very useful feature for those who burn midnight oil and want to get messages sent off first thing in the am. Or for those who want to save something and polish it up later. Slack has this as well.