This is probably preaching to the choir, but hosting your own FOSS chat is nowadays a very viable way to avoid being dependent on a centralised service like Slack. Your options include:
* Riot.im / Matrix.org (decentralised global network; e2e encryption; open protocol)
* Rocket.Chat (Meteor-based; focus on UX and feature)
* MatterMost.com (clone of Slack UI; open core license)
* Riot.im / Matrix.org (decentralised global network; e2e encryption; open protocol)
* Rocket.Chat (Meteor-based; focus on UX and feature)
* MatterMost.com (clone of Slack UI; open core license)
* Zulip.org (all about threads!)
* ...or indeed IRC or XMPP.
(disclaimer; I work on Matrix).