You are probably thinking of either Dendron or Foam.

Thanks for mentioning these! They look pretty interesting.

You're right - dendron. Thanks

No, they have no specific vscode-extension. That would kinda defeat the whole purpose of it. But there are a bunch of markdown-extension doing more or less similar things. But IMHO a dedicated app serves the purpose better so far. VS Code is good, as an editor. But it's not (yet) an app-platform.