Just to give the perspective of the community team at MDG:
We looked into creating an official community slack channel a few months back and decided that it didn’t make sense.
This article highlights some of the reasons why: http://blog.freecodecamp.com/2015/06/so-yeah-we-tried-slack-and-we-deeply-regretted-it.html
After this article was published (and perhaps in part because of it), Slack limited team size to 1,000. We have thousands of users in this forum, and didn’t want to create an unsustainable slack team which we would have to figure out what to do with down the line.
So that’s the current situation with meteor and community chat options. IRC is currently a community-driven alternative for those who prefer synchronous communication, as well as some of the unofficial slack channels mentioned above.