Odd this is the first time I have heard this request, it does sound like a reasonable request to me, but given it is so rare I do not think it is going to find its way into the product for many years.
As it stands your 2 options are
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Write a simple plugin that introduces this, would probably only be 10 lines of code. You can commission it on Marketplace
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Archive topics where you want to disable deletion. End users can not withdraw content from archived topics.