They key missing ingredient here is "ownerhsip" you can debate all the pros/cons of Product Manager functions and even Project Management functions till the cows come home, but regardless of functional skills / job description, the only key for success in any of these roles in true
Product Ownership.
and I'm not talking about hiring a PM off the street then bestowing him/her with the "ownership" of a product they've never seen before! True Ownership comes from a self generated interest in the well-being of the product, its customers, the technology empowering it and its sustainability.
This does not happen overnight, and is certainly not a skill that can be taught.
The most successful PM I've worked with over the years had one thing in common: they actually CARE about the product AND about the users (clients). this generates an internal drive for excellence like no other, in both satisfying business goals, client asks, and technology roadmap, even though those three are almost never on parallel lines.
As to the original question, where did they all go? I don't believe the role in it self went away, but rather smart companies nowadays recognize that a set of skills and an Agile/SCRUM/PMP/etc certificate will not make you a "drop in" successful PM, but rather they rely on the internal champions of the products to help shape the story.
This means more developers are stepping into the role, and through the right level of support they can/are picking up the business/accounting/client side of managing a technology product.
Also, an important part of a Product Manager (Owner) role is relationship management and communicating with people (clients, developers, business owners, sales, marketing, support, design, etc ...).
Human interaction is unfortunately another one of those things that cannot be taught! communicating with people is so different from sending status update emails, or adding them to auto notifications from tickets in a feature tracking system.
Sadly, most of the PM I've worked with and from what I've seen, the industry actually encourages the automation, over the human interaction. yet another reason why this role is changing and is being redefined.
just my 2 cents.