Your deleted drafts will be still saved for 60 days in the Deleted draft page. Once a draft is deleted, it will unlinked with any of its child drafts. But its sub tasks subtasks will be not unlinked.
Deleted drafts
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Delete Subtask
At any time once a sub task is deleted from its parent draft, it will be permanently removed. You can not find it in Deleted draft page. Subtask can only exists under its parent draft.