Just came across this one. Haven't tried it yet. It looks like it's using Python, so installation might be more involved than usual:

"Outspline is a free and open-source modular outliner whose functionality can be extended with addons. The most important addon is Organism, which adds advanced time management features and turns the application into a personal organizer, perfectly suited for working with todo lists, scheduling tasks and reminding events."

https://kynikos.github.io/outspline/index.html

Screenshots here:

https://kynikos.github.io/outspline/screenshots.html