We added this script in T221112: Document how to convert projects into subprojects/milestones etc (sudo privileges for phab admins to run move_project script).
We should not use this script and thus should not perform this action. The script

• does not remove parent project tag when project to convert becomes that parent project's subproject
• has an unknown bug which makes projects inaccessible, see T177787#9023514, which also made a milestone disappear from a workboard

I don't like creating DB inconsistencies and making stuff inaccessible. Thus

• Remove https://gitlab.wikimedia.org/repos/phabricator/phabricator/-/blob/wmf/stable/scripts/move_beneath.php
• Update https://wikitech.wikimedia.org/wiki/Phabricator#Converting_a_parent_project_into_a_subproject
• Update https://www.mediawiki.org/wiki/Phabricator/Project_management#Converting_projects_into_subprojects_/_milestones