Followup from T221112:
In T195855#5190491, the command created data inconsistency.
For example, T57111 was in both #Pywikibot-other-scripts and #pywikibot-solve-disambiguation.py or T57020 was in both #Pywikibot-other-scripts and #pywikibot-weblinkchecker.py. The script does not check if a task is in both the future parent and the future subproject, to remove the future parent project when turning a project into a subproject.
Affected tasks were: https://phabricator.wikimedia.org/maniphest/?ids=104952,105581,57111,57020,167709,85334,117176,57048
A task cannot be in both a parent project and a subproject of the same parent project, so tasks like https://phabricator.wikimedia.org/T85334 have invalid associated project data now.
I think there's currently nothing else left for me here? :)