Personally I would like a Slack that works on older Mac operating systems. I had to leave the groups I was in since Slack made updates last month, and now I need Mavericks or newer to continue using it.
Yeah, I've never even installed the desktop client. It's an Electron (JavaScript) distribution, so it's essentially just a self-contained version of the site, isn't it? Just go to *.slack.com and you don't have to worry about it.
Having an alternative that works on older operating systems wouldn't help you unless the others in your group want to switch just for you. Network effects win.