plone.app.workflowmanager provides a super useful feature IMO. I updated the package in recent years every now and then to show it to clients for pitches. I also tried to clean up the branch mess and make a proper release. Lots of people put effort into trying to upgrade the package to Python 3 and stuff. Though, to be honest this feels like a lost cause to me and I would rather recommend starting over from scratch.
I started to write a workflow manager in JavaScript a few years ago (actually multiple different approaches) and there are lots of modern JS libraries that allow you to create graphs. Though the workflow use case we have is non-trivial and it is not easy to find a library that does everything we need.
I would love to see some initiative (maybe a GSoC) project that creates the necessary restapi endpoints and then builds a modern solution on top of that for Plone 6 / Volto. In theory, such a solution could be backported to Plone Classic but I would not recommend it. Making modern JS libraries work with an outdated JS stack is one of the worst tasks one could choose (and yes, that's still true with the updated JS stack we have for Plone 6 classic, we still have jQuery and other ancient libraries in there that will cause pain and will not work with React or any other modern Framework that needs control over the DOM).
I'd be happy to give some guidance if anyone would want to approach this.