Hi @anon59828738, thanks for reaching out!
This is a good questions! You can create a private project to save those tasks instead of deleting them from Asana. By doing so, you will be able to use them in the future if you need it!
You’ll simple need to multi-home them in the private project and then remove them from the original project. You can also select several task at the same time with the multi-select feature.
Alternatively, you can also add custom fields or tags to the tasks so you can have a better overview of the tasks that are “not needed” or “not doing”.
You are right, it’s currently not possible to mark a task as “not needed”, “not doing” vs “Completed”. You can add a vote to a similar product feedback here: Reference-only Tasks that can't be marked complete
I hope this helps! Let me know if you have any follow up questions 