The "no-source-code" license label is not a license and must not be used on its own. See the tracker bug 615456 for a longer explanation.
I wrote a mail to upstream.
> Could you clarify under what terms slack package is distributed?
> Does slack have some End User License Agreement besides Term of service, User Terms of Service ?
> Emma Busch (Slack)
> Apr 28, 1:25 PM PDT
> Hi Vladimir,
> I'm really sorry for the confusion and thank you very much for your patience.
> The Linux desktop beta is Proprietary Software that's distributed under our Terms of Service which means that you won't be able to redistribute the app for another distro. That being said, you can create a script to install Slack on Gentoo under any license you want because the script would be yours. I'll also share that unknown license bit with our engineers to investigate.
In slack package for Ubuntu/Fedora I have found this:
# grep -A2 email@example.com ./usr/lib/slack/LICENSE
So, upstream doesn't have a license for slack package itself, just a ToS - https://slack.com/legal
Since their terms of service don't mention the software at all, I suggest that we label the package as LICENSE="all-rights-reserved" and RESTRICT="mirror bindist".
Pull request created https://github.com/gentoo/gentoo/pull/4533