[smtddr]

I don't know how "proprietary" or "walled-garden" it is, but I can connect to slack using open-source "irssi" on linux.

http://www.tricksofthetrades.net/2015/09/10/slack-irssi-conn...

I feel like that has to diminish the walled-garden, proprietary weight at least a little bit if slack & irssi can communicate. And it works over SSL too. Once I started doing that, I didn't mind using slack. You have to "/ignore" certain server messages though or your whole screen will be spammed with them and you'll barely see the stuff people are saying.

[nfd]

Slack may have a working IRC bridge, but it still has a closed-source server. You're still trusting a third-party with all your data.

[oceanstone]

Also the IRC/XMPP bridge has to be specifically enabled by a team admin. After this, the bridge can be disabled at any time by team admins.

https://slack.zendesk.com/hc/en-us/articles/201727913-Connec...