This YouTube video demonstrates features in next-gen project, which also shows creation of sub-tasks in next-gen project (2:20). However the feature to add sub-tasks is not available in the instance.
- Create a new Scrum/Kanban next-gen project
- Add an issue to the board in this project
- Click on the issue to be viewed in new issue view (popup screen)
- There's no feature to create sub-task.
- Sub-task is not available in Project settings > Issue types page, and can't be added.