Expose message edit histories
If users are given the ability to edit their messages, it is useful, at least for moderators and admins, to be able to see what their previous messages were. This is so that disputes between users can be resolved, where a user responds to a message that is subsequently edited to change the context and meaning of the response.
I came to respond to this because although this is a professionally-focused application, this feature can be used to great effect as a gaslighting tool.
Consider the HR nightmare of an employee messaging another with suggestive or aggressive text, and then editing the previous message to read differently. Subtle enough, and the other person can easily be made to believe they are misreading the situation, or the message.
This might sound like a bit of an extreme scenario, and maybe it is, but it happens. We should at the very least expose that a message was edited; better to expose in what manner.