[lostcolony]

I agree with you that for your final sentence it's optional. That said, I still do it.

But that's kind of ancillary to the point of this post, no? The main point it's trying to drive home is be as informative as possible. Don't assume your audience knows your tone, your conventions, your abbreviations.

I suspect this is largely for those who have newly entered the work force, who may only know texting style communication (with friends, shared corpus of knowledge), and formal business communication, and this is to warn them that Slack, while less formal than the latter, doesn't imply the shared context that the former would often have.