It looks to me like they aren’t saying the application will no longer run on Fedora. They are saying they no longer provide official support for Fedora. This is the same situation many other Linux distros are in already.
haven’t used slack with alot of group or whatever they call it but you can always use snap if they stop providing update.
According to the offical EOS announcement, it seems like snap will also stop being updated:
If you’re using a Windows, Mac or Linux OS that has reached end of support, you’ll still be able to access Slack as long as you’re using a supported app version, but you won’t receive app updates. Slack won’t be available to download from the App Store, Microsoft Store or Snap.
Slack has really followed the MS model. First, you could use them with any irc client. Then they broke that, though it can still be used with weechat using the wee_slack plugin. They seem to get worse and worse, but as most companies use Windows, they get away with it. I hope that some other company comes along and is as nice as Slack pretended to be, allowing irc clients and so on, without tying you to their bloated apps. Right now, all I can say is try to get your company to use something less Microsoft-ish.