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.
In the markdown editor you can sort lines alphabetically, a completed checkbox is after a completed checkbox alphabetically so that should work for you. You'll just need to highlight all the lines with checkboxes, and then the keybinding should be Ctrl+alt+s if I remember correctly.
I know this feature was requested many eons ago but I recently started with Joplin and realised this would be a very useful feature to have so have made a plugin which does it: