There doesn't seem to be any setting to do that. I guess you'd have to find the places where the post count is updated when a post is deleted and change it to check for soft deletes. Searching for set('posts' finds a couple places, but I don't know if that's all of them. There's also admincp/misc.php where it rebuilds post counters, I think you'd have to take out the "AND visible = 1' from the SQL.
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