Double Zero,
"
this all seems to refer to modern airliners with bags of power"
Not quite, it was well established in the sixties when I first became acquainted with it, aviation historians may be able to shed light on the time when the regulations required obstacle clearance following engine failure at all phases of flight. Of course, the regulations have been refined over the years, but the basic principals remain the same.
If you want to go even further back, to a time when obstacle clearance was not considered, but guaranteed One Engine Inoperative performance was, you'll have to go right back to the DC1 and DC2. It was TWA's specification that the aircraft must have guaranteed engine out performance. Millions of passengers in the 80 or so years since then have a lot to thank Jack Fry for.
The "modern airliners" from the sixties are, with a few exceptions, now only found in museums. The sixties are, after all, going on to half a century, hardly modern times.
Best Regards,
Old Smokey