Notion is not available as a self-hosted solution but there are plenty of alternatives for power users and business that want to host a solution on-premise. The best Self-Hosted alternative is Standard Notes, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Notion and loads of them is available as a self-hosted solution so hopefully you can find a suitable replacement. Other interesting Self-Hosted alternatives to Notion are TiddlyWiki (Free, Open Source), TagSpaces (Freemium, Open Source), MediaWiki (Free, Open Source) and Redmine (Free, Open Source).
MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia. It is now used by several other projects of the non-profit Wikimedia Foundation and by many other wikis, including this website, the home of MediaWiki.
OpenProject is an open-source project management software with a wide set of features and plugins. It supports project timelines / Gantt charts, issue tracking, wiki, document management, time and cost reporting, code management, Scrum and much more.
Kanboard is a simple visual task board software. Kanboard is a free and open source Kanban project management software. There is no fancy user interface, Kanboard focus on simplicity and minimalism. The number of features is voluntary limited.
Etherpad is a highly customizable Open Source online editor providing collaborative editing in really real-time. Collaborating in really real-time No more sending your stuff back and forth via email, just set up a pad, share the link and start collaborating!
Dendron is an open-source, local-first, markdown-based, note-taking tool built on top of VSCode. It supports all the usual features you would expect like tagging, backlinks, a graph view, split panes, and so forth.