In the past I'd recommended Trilium Notes as a good FLOSS alternative to Obsidian for a note taking database app, but unfortunately today after failing to restore a trilium backup file to a new install, and finding that the documentation on backups isn't accurate (as in, there are not buttons in the app that the documentation claims are there, and nothing I have tried has worked to restore the backup), I discovered on the github a Claude.md in the repository, and the commit history shows that in the past few months Claude has become one of the most prolific contributors on the project.
If anyone is using this app based on my previous recommendation, I'd personally suggest migrating to a different note-taking app that isn't vibe-coded (and exports backups to plain text, instead of some database format), perhaps CherryTree?
I've been a Joplin user for years, looked at Trilium once or twice, but the complexity and (iirc) database-backed storage kinda kept me away. The latter also always slightly irked me about Joplin, but at least you can export the whole notebook to markdown.
Which I did, when Joplin declared slop contributions OK (and a bunch of plugins were also on the slop teat, and you kind of need plugins). Then I started up SilverBullet and I haven't looked back.
In terms of "I want an alternative for Obsidian" I think it might actually come closest.