Are there any open source Notion alternatives? This is the main benefit of open source software, in my mind: no one can "take" it from you because it doesn't belong to "them" in the same way that a product does.
BookStack doesn't have any client apps — it's basically a PHP app like WordPress that you can self-host, but the collaboration feature set is pretty good. It's a wiki that feels a bit like Notion.
It is BSL licensed, the only restriction is that you cannot run a hosted version for other organizations to use (aka compete with the only way the project maintains itself).