I contacted the support already, but they can’t help.
This is very, very strange. I contacted the slack support 2 times (for very minor and trivial things) and they’ve been nothing but amazing. Probably one of the best support experience I’ve ever had.
But besides that I also have a problem with Slack being a very closed platform.
This is an interesting point. It never occurred to me to think at slack in those terms probably because I don’t really care about the “content” on that platform and the only thing I’m concerned is that if the service works or not. And as for the rest, slack is one on the most open platform I have ever used since can be integrated with basically everything.
The 10.000 messages limit is another issue.
With this I totally agree but again, this is only a problem if one wants to use slack with a long term goal in mind. I was thinking more at something 100% extemporaneous, for quick chat related to problems one is facing while working on something and not to store long solutions and discussions about complex problems.
Another thing I totally hated about it, is how they handle account switching. It works quite well in the apps, but it’s a nightmare in the browser.
I always used the standalone app but now in the browser switching teams can be done using this menu. Not sure if has always been this way or not though 

IMO the best solution would be a real time chat directly here on the forum but since it’s not available I think a tool like slack could be a decent replacement.
The only reason why I think slack is better compared to gitter is the level of structure one can have because, and correct me if I’m wrong, gitter doesn’t have 1) direct messages and 2) multiple rooms (although I’m not 100% about the last one, I think there are multiple rooms available)
The main reason while I was asking about this is because sometimes a forum is not the best tool to solve a issue.
And as I was saying yesterday I 100% think the forum should NOT be replaced by a real time chat. The form is super helpful at the current stage.
Anyway, always nice to hear opinion about how can we improve things 