Well, for starters, “older fandoms” + “slash” makes me think of Western fandoms, which did not use this ordering business all that commonly even in the cases where they had strong roles.
People who intentionally write Sasunaru and Narusasu as different ships know which one they’re writing regardless of what kind of sex is present or not. It’s about characterization and author preference. They’d put Sasunaru on their fic because they think of themself as a Sasunaru and not Narusasu writer.
People who didn’t want to write strict roles said so, sometimes with “A x B x A”.
