In a literal sense, I suppose this is true. The subject matter in the sense of "the stuff in the world" remains "the stuff in the world" but not only has the stuff in the world changed in the last 150 years, the way we treat the stuff in the world photographically has changed enormously.
In the first, say, 50 years of photography it would never have occurred to anyone to take a closeup photograph of an eye. Do we claim that the subject matter is the same, because, well, it's basically just someone's face? Whether we choose to split hairs on what "subject matter" means or not, something has changed.
A century ago, post mortal photography was usual, it isn’t today.
In the seventies, certain pictures of naked teenager girls where considered as art, it isn’t today.
Not so long ago wedding pictures where expected to show the golden rim around the day, nowadays a straight forward reportage of the day is preferable.
Etc etc
If you consider a person as a subject, didn’t it change?For sure society did.
The picture as such is not a main goal for me, it’s what it describes on a not literal way. It’s about peoples and what is going on around peoples, peoples are even not necessarily in the frame to search for the circumstances where peoples live in.
If a picture can be a reflection of social circumstances, it better is not ‘good’ or ‘beautiful’ or according to whatever rules. It better is what it is.