the plugin SubtasksOnBoard has been archived last year. It was never part of the Kanboard plugins repository.
I find that very useful and asked the author if i can take over. He agreed. So i made it compatible to Kanboard standards (hopefully), tuned the css, removed some glitches and tested it with ThemeMaestro.
The repo for the json for Kanboard is changed. Pull request ongoing. After that it should be available via Kanboard plugin manager in the app.
Thanks to @creecros for an enlightening code session!
Have fun, Fx