Thanks for the link and man did I just read through some history…
It was originally written in Java by Paul Falstad who provides it as an applet here: Circuit Simulator Applet
With permission Iain Sharp put in just over a month of work converting the project into HTML/Javascript using Google Web Toolkit (GWT), licensed it under GPLv2 and placed it on his github repo ~8 years ago.
Work on the project has been pretty steady since then including commits from Paul Falstad himself. Though activity seems to have dropped to near zero in the past 2 years. On Ian’s site he says,
“Time is in short supply right now, so I am no longer regularly updating this page. For the latest version please to go to Paul Falstad’s page.”
Looking at Paul’s fork commits are still constant so Paul’s version appears to be the one that’s actively maintained and it’s available on his website here.
It’s quite a story of how someone stepped up 8 years ago to help move a Java project to HTML/JS under GPLv2 which he maintained along with the original creator and others for 6 years until the original creator became the primary maintainer.
tldr;
Actively maintained interface is here: https://www.falstad.com/circuit/circuitjs.html
Thanks for looking into that and getting the additional info including a maintained project link.