See https://github.com/cjbarber/hipchat-alternatives for more ideas.
Besides a gazillion open source wiki projects…
For a bigger far more all encompassing documentation toolchain:
Jive Disclosure: former employer (but thoroughly recommend for the right culture/environment)
Salesforce.com’s Chatter Disclosure: former employer (don’t recommend based on user experience 3+ years ago)
As for the email from today, that Atlassian ends the support for self hosted server.. most important site right now.
But as far as I can see, no real alternative (with plugins) for Confluence.
https://backlog.com/ - a replacement of Jira + Confluence + Bitbucket.
We recently switched from Confluence to AllAnswered and really loved it. AllAnswered combines Wiki documents like Confluence and Questions & Answers similar to Stack Overflow. Great product and very easy to use. Here is their website https://www.allanswered.com
Check out Drupal Wiki as Confluence alternative. Can be self-hosted (on premise) and has almost the same structure and functionality as Confluence - even content migration from Confluence is possible see Migrate from Confluence to Drupal Wiki.
Check out Doctave if you're looking for a developer-friendly Confluence alternative that uses a docs-as-code approach. You can also use the open source documentation site generator for free. (Disclaimer: founder here)
Jira alternative (on-premises and open source) --> OpenProject (https://github.com/opf/openproject)
I would shamelessly add my project, Docmost, as an open-source alternative to Atlassian Confluence, with real-time collaboration.
Github: https://github.com/docmost/docmost
Website: https://docmost.com
Ticketing? Of course you are missing Azure Boards!