Interesting debate. Here is the thing, being an old brawd of 50, haha, I can tell you that the older cookbooks and recipes, when they called for butter, well it generally meant salted butter - that would cover a time frame from around the 1950's until approximate 1990. But on the whole, a good cookbook should really have a section where they tell you what they mean when they call for butter, the size of eggs used in a recipe etc.
Cookbooks written during the first and second world war, generally we made with the idea of using cheaper more available ingredients.
Recipes and the ingredients called for, change with the times. At one time, the size of eggs used was medium, then at one time extra large and now generally the size used is large.
From about 1990 until present, well generally a lot of the cookbooks and recipes do call for unsalted butter and frequently also unbleached flour. This is a trend.
In Canada, it will soon be illegal to sell fat products that are composed of transfats, so we will see a new trend again as some recipes are adjusted.
It is generally wise to use what is called for and if a recipe calls for salted or unsalted butter use what is called for. If you cannot, then adjust the salt called for in your recipe to accomodate the salted butter that you are replacing the unsalted with and vice versa. This generally will not make any noticable difference in a recipe.
The longer you are around this earth you start to notice that cooking becomes very trendy with ideas catching on from different parts of the globe.
But honestly you cannot assume that a recipe means unsalted butter unless it states so and vice versa. It really isn't so much definitive as it is a sign of the times. Presently, newer recipes are likely being made with salted butter - but not always.
Hugs Squirrelly Cakes