I mean the old rocket landings threw away 90%+ of the rocket first. Minus the hot staging ring and some molten metal almost all of this rocket was present at both its landings.

The flying bedstead of 1964 was a reusable rocket that propulsively la ded and was reused. It didn’t go very high tho. . .

https://www.nasa.gov/image-article/flying-bedstead/

No it wasn't. It was powered by a jet engine. Specifically, a GE CF700.

https://en.wikipedia.org/wiki/Lunar_Landing_Research_Vehicle

I wonder how much earlier a rocket like Falcon 9 (with automatic landing of the lower stage) could realistically have been developed and built. In the 70s? 80s? 90s?

I think even into the 90s it would be difficult from a compute and sensor perspective.

Rocket landing on the moon does not count? Luna 9 in 1966 was the first spacecraft to achieve a controlled soft landing on the Moon.

70s no. 80s maybe, depends on clever use of computing. 90s yes.