Always show Apple Mail Messages as unopened?

Unfortunately, no. Tell Apple what missing features you'd like restored or new features added to the system via Feedback - macOS - Apple.  

chredmond wrote:

yes, thanks, understand that ... am looking for a setting that makes messages always show as unread automagically, whether or not the messages were opened.

If there was a setting, I would have mentioned it.

You asked for a way.

Mark them as unread when you finish with them.

not to be too pedantic here bc hey who cares really ... but I DID ask how to set a preference, not just a way. That's why I worded my message saying just that.

Valid. I didn't really get that far.

In the meantime don't hold your breath.