I am constantly, and I mean constantly finding subpanels in homes wired with 3 wire feed - meaning no 4th conductor - meaning neutral and ground are shared and bonded in the subpanel just like in a main service panel, and the subpanel is in the same building as the main panel. To my knowledge it was never acceptable to wire a subpanel in the same structure as the main panel without running both a grounded and a grounding conductor and keep them isolated. The only NEC allowed exception I am aware of is if its a separate structure not connected to the home in any metallic means, and it must have its own grounding electrode system (article 250).
My problem is perpetuated by a multitude of electricians that constantly claim these installations are “grandfathered” by the NEC. I don’t think any of them own an NEC handbook that goes back to these dates though… and neither do I.
So, I would absolutely love it if somebody on this forum happened to have an NEC code book from say, 1975 or earlier, and could possibly scan the page covering this topic and post it here. I would greatly appreciate it.