I regularly see bone marrow on restaurant menus, and frozen cut bones are available in grocery stores.

I've lived in North Carolina, Virginia, Ohio, Texas, New York and Colorado.... Wherever you happen to be I'd guess that if you start looking you'll find that it's always been there.

Just not on menus of restaurants the 90% of the people go to regularly...

The admonitions about giving dogs bones derive from fast eaters tending to choke on them - not the bone's nutrition. (Although it would not surprise me if somehow the idea has gotten corrupted)

Some bones, especially chicken bones, also splinter and cause all kinds of serious havoc on the digestive system.

I've had pork bones break in scary ways as well when the dogs got into them. I personally don't give my dogs anything but rawhides and large beef bones anymore, both of which you just have to buy specially really.

On the subject of actual marrow, it adds a lot to soups and stews. Usually added via just cooking the bones in the soup/stew and removing towards the end.

Cooked bones. Raw chicken bones are bendy and harmless.