Well, I wouldn't use such a clone. When Slack is down, I actually feel relieved. I know I can turn off Slack any time I want, but the reality is that companies expect one to be online all the time (except during breaks of course) in regular working hours. I try to keep Slack as an async communication tool, but stakeholders just don't care about that. Also, I have a bunch of channels I cannot mute, so there is always something going on on such channels (which brings anxiety and discomfort).

So, when Slack is down, it's actually a nice "everybody is offline: either take some time off or work focused on whatever you need to work on"... which is a nice thing to experience from time to time during regular business hours.