I disagree with some of the things in this article (mainly that IRC is necessarily better for your company) but Slack isn't ideal for FOSS projects. It's a very long way from ideal.
Main reason being: Slack is a company and as such they can change their policy at any time. There isn't any specific right granting FOSS projects any status. Slack can ban the users they want, can deny access to accounts they want, they can be sold to other companies and can be bankrupted, bring it all crashing down.
Slack isn't a tool, it's a service. It's not in any way whatsoever like a debugger or IDE, even if those may be closed source. A tool is something you have and can use. You might not own its manufacturing but you own its potential use. Like a hammer you didn't make but is yours to do anything with. You can use it a million times if you want.
Slack is someone else's hammer that you pay to use. That's a service. The fact that they allowed you to use it for free for some undefined period of time doesn't mean you own it or dictate any rule of its use. It's still their hammer. They can stop you from using it at any point in time.
FOSS projects are supposed to be inclusive, open, transparent and fully decided by an owner or a community. Giving away control to a third party with no contract whatsoever is a terrible idea and a good way to break many of those premises.
Slack is being used for FOSS because of the failings of IRC noted by others in this thread, but that doesn't mean it's a good idea. Certainly not when the audience for Open Source is already very accustomed to IRC.
Slack is great for the turnkey solution it is, it offloads that IRC configuration you'd have to do, and it's easier to introduce non-technical people to. It's good for paying customers that have a clear contract with the company and as such gain certain rights. It's not, in any way, ideal for other cases. It might fit them, but it's not a great idea.
Main reason being: Slack is a company and as such they can change their policy at any time. There isn't any specific right granting FOSS projects any status. Slack can ban the users they want, can deny access to accounts they want, they can be sold to other companies and can be bankrupted, bring it all crashing down.
Slack isn't a tool, it's a service. It's not in any way whatsoever like a debugger or IDE, even if those may be closed source. A tool is something you have and can use. You might not own its manufacturing but you own its potential use. Like a hammer you didn't make but is yours to do anything with. You can use it a million times if you want.
Slack is someone else's hammer that you pay to use. That's a service. The fact that they allowed you to use it for free for some undefined period of time doesn't mean you own it or dictate any rule of its use. It's still their hammer. They can stop you from using it at any point in time.
FOSS projects are supposed to be inclusive, open, transparent and fully decided by an owner or a community. Giving away control to a third party with no contract whatsoever is a terrible idea and a good way to break many of those premises.
Slack is being used for FOSS because of the failings of IRC noted by others in this thread, but that doesn't mean it's a good idea. Certainly not when the audience for Open Source is already very accustomed to IRC.
Slack is great for the turnkey solution it is, it offloads that IRC configuration you'd have to do, and it's easier to introduce non-technical people to. It's good for paying customers that have a clear contract with the company and as such gain certain rights. It's not, in any way, ideal for other cases. It might fit them, but it's not a great idea.