This isn’t something that I do much myself, but two ideas are:
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Save your TaskPaper document somewhere that does version control. The Finder can do this (File > Revert > Browse Previous Versions), or for more robust and predictable history use something like github. Benefit here is you can delete outdated tasks, but have a way to recover them if needed.
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If you want to more easily find all these outdated tasks in one place then I would suggest creating a separate “Outdated” TaskPaper document that you move outdated tasks into. If you wanted to automate this somehow there are scripts for moving tasks between documents linked to from the extensions page: