AFAIK this doesn't exist in Joplin, but correct me if i'm wrong. Couldn't find this.
It's possible to sort the notes-list by putting the completed tasks at the bottom. Inside a note you can add also checkboxes. I often use one todo-note for a week or (small (sub))project.
It would be handy to sort the text inside a note where all the completed tasks are moved towards the bottom.
Maybe a plugin who scans every line for "- [x] " and put this line on the bottom? I don't have the skills to do this by myself.