Q: The Chicago Manual of Style says “Ms.” should have a period, but Gloria Steinem and the other feminists who popularized the term don’t use a period. Which is right?
A: In matters of style, there’s no absolutely correct or incorrect call. In grammar perhaps, but not in style.
Most American stylebooks, however, will advise using a period for an honorific, with the exception of “Miss,” which is not an abbreviation. One might argue that “Ms.” isn’t an abbreviation of anything, either. But the fact remains that it is not a noun on its own, and exists only as a courtesy title before a name.
It’s worth noting that in Britain, “Mr” and other titles are used without periods. That’s a peculiarity of British style.