AppFlowy is the closest open source stand-in for Notion: documents, boards and databases in one workspace, running on your own machine or server. The trade is maturity and the ecosystem around it, not the core idea.
What you are replacing
Notion's value is that documents, tables and boards are the same object viewed differently, plus a hosted service nobody on your team has to maintain. Any replacement has to answer both halves, and most open source projects answer only the first.
It targets the same shape directly: pages that hold both prose and structured views, kept in one workspace rather than split across a wiki and a spreadsheet. Because the data sits on hardware you control, it also answers the question that usually starts this search, which is where the company's notes actually live.
What it does not replace
The integration surface is much smaller, and templates and third party blocks are where the difference shows first. A team that leans on Notion's automations and connected tools is replacing more than an editor.
Stay on Notion when the team is non technical and nobody owns the server. A self hosted workspace moves the maintenance from a vendor to a person, and if that person does not exist, the migration has not saved anything.