Not unheard of, but also I do think it would be different - everyone knows holiday time is limited, which feels different to having an unusually long weekend every week. It shouldn't, and maybe I am overly concerned by the impact this would have - but I would still be much more comfortable with the idea if it is at least available org-wide rather than by exception only.
This sends us off on a tangent about communication techniques, and specifically the down-sides of Slack. I personally try to use it like a richer email system (i.e. asynchronously as much as possible) but you always get the odd person who insists on "hello are you there" as their opening message.
