I’ve had this claim go both ways. I had at least one history teacher in highschool who strongly argued for this rule applying only to plural possessives. As with many rules, consistency matters more than anything else. I think that only applying it to plurals makes more sense (in that the distinction there actually helps you keep track of the number of the word) but it seems that nowadays it is more common for people to use s’ construction for any word ending in s.
I’ve had this claim go both ways. I had at least one history teacher in highschool who strongly argued for this rule applying only to plural possessives. As with many rules, consistency matters more than anything else. I think that only applying it to plurals makes more sense (in that the distinction there actually helps you keep track of the number of the word) but it seems that nowadays it is more common for people to use s’ construction for any word ending in s.