- Jun 2021
Ward Cunningham may have been using a similar UI prior to it for other projects, but he unveiled the Smallest Federated Wiki at IndieWeb Camp 2011 in late June: https://indieweb.org/2011/Smallest_Federated_Wiki. I don't have a receipt to prove it, but I have to suspect that Andy's version was certainly influenced by Cunningham's work.
Mike Caulfield, subsequent author of the influential The Garden and the Stream: a Technopastoral, Iterated on the Smallest Federated Wiki and created a WordPress-based plugin shortly thereafter called Wikity that used some of the card-based UI that Obsidian comes with out of the box.
Both had some early influence on the UI-based research that the IndieWeb space has done since. For those interested, there's also a sub-group within it focusing on digital gardens, commonplace books, Zettelkasten, etc. that can be found here: https://indieweb.org/commonplace_book