Q. Prepositional phrases beginning sentences. No longer followed by a comma?
A. For starters, it isn’t one-rule-fits-all. After a short phrase a comma usually isn’t necessary. After one that’s especially long or whose syntax involves more complex elements, it’s usually a good idea. But as you can see from these sentences, whenever a pause is intended, a comma does the trick. See CMOS 6.31 for more examples.