Not sure where you're looking but source code is here (not sure why they don't link to it?) and last update was about a year ago and there are recent commits.

https://github.com/alainm23/planner

I just discovered this and really like it, it's a huge improvement over GNOME Todo. My only complaint is that it seems to choke on a large number of (completed) tasks. Though that might be more of an issue with Nextcloud not clearing out completed tasks and a lot of caldav apps have this problem.