When you use Slack, you don't "use it in a web browser", you use the (proprietary) software, delivered by Slack Technologies, which happens to run in a web browser. The fact that the web browser is itself free software is as irrelevant as the fact that the kernel and user-space services supporting skype on linux/bsd systems are open source. The actual software, everything that actually relates to the task at hand (group communication), is non-free.
Actually it's not completely irrelevant, as the fact that the host platform of proprietary software is free software has value in the security-based argument in favor of free software, but that's only tangential to the topic at hand.
Would it also be bad to host the project's source code on GitHub
Not necessarily, if really all you're using github for is hosting, then there's nothing wrong. That is, if you don't use their issues/PR system or other auxiliary services. This is how, for example, the linux project uses github. In this case, all that you're asking people to do is use git (free software) to download some stuff from servers owned by Github, Inc.
Running the website on IIS is fine though; this imposes absolutely nothing on your users or contributors.
But just visiting the project page on GitHub downloads and executes proprietary JavaScript. If that's a problem for Slack, why isn't it a problem for GitHub?
You don't have to visit Github's web ui at all if you're just using it as repository hosting. At least users and contributors don't have to, the maintainer needs to occaisionally for configuration reasons.
Also it's worth noting that github's entire UI, at least for browsing (I didn't try creating an issue or editing a file), works just fine with javascript disabled.
Don't all contributors have to at least set up an account through the web interface?
Even if you can connect to GitHub without ever touching their proprietary JS, that's clearly not the typical route. Just like you can connect to Slack using nothing but open source clients, but one is "enticed" to use the proprietary client.
> Don't all contributors have to at least set up an account through the web interface?
You can clone/pull anonymously from github and eg: email patches.
I'd say using github just for hosting read-only/read-mostly git is a bit like running http on IIS or email on Exchange - or indeed a propietary XMPP server that allows federation.
I'd prefer Free projects to use Free infrastructure - but building on established open protocols is good. Contrast with bitkeeper: you'd have to reverse engineer the protocol to pull/push - or with slack: there's no way to set up your own server snd federate with it.
Actually it's not completely irrelevant, as the fact that the host platform of proprietary software is free software has value in the security-based argument in favor of free software, but that's only tangential to the topic at hand.
Would it also be bad to host the project's source code on GitHub
Not necessarily, if really all you're using github for is hosting, then there's nothing wrong. That is, if you don't use their issues/PR system or other auxiliary services. This is how, for example, the linux project uses github. In this case, all that you're asking people to do is use git (free software) to download some stuff from servers owned by Github, Inc.
Running the website on IIS is fine though; this imposes absolutely nothing on your users or contributors.