Hi @Florian_L_Hermitte, thanks for reaching out!
You can create a report of subtasks that you would like to delete. Please find more details in this article. You will be able to search by subtasks selecting the Add filter > More.
I also recommend you checking this great post with more tips and workarounds to delete projects/tasks/subtasks that are no longer needed: Best Practices when deleting a Project
I hope this helps! Let me know if you have any other questions 