It's almost always better to use the process name versus the application bundle name, because the latter can be spoofed by a crafty user. Just renaming Slack.app to something like "slck.app" would bypass your Restricted Software setting if you have it looking exactly for "Slack.app"
The one caveat to using the process name is that it makes deleting the application difficult or even impossible, at least from within that Restricted Software title. Because it will only delete what you tell it to, so deleting "slack" would only remove the executable, and not the entire application. It renders it unusable, but isn't going to completely rid the app from the machine.
As for why your Restricted Software settings aren't working, have you confirmed that the setting was pushed to the Mac you're testing it on? It's not immediate like a Config Profile. The Mac needs to check in before it will get any newly set up restricted software settings.
