Why did my plaster walls stop half inch above floor and expose room to wall cavity?
Old houses that had interior plaster as a finish had what they called a plaster stop to control depth of the plaster. This was usually a strip of 3/4 wood at bottom of wall and around doors and windows. It looks like this strip was removed but normally it was always left in as a nailing strip for the baseboard. Your application of the foam is good idea.