There's plenty examples indeed if you just dig around. I have a colleague who published a theoretical paper [0] in one of the top fluid mechanics journals 5 years back. That paper originated from his lecture notes in a course that he gave for the first time, where in one part he followed the classical theory of waves behind a ship done by Lord Kelvin and others back in the olden days. Then he thought "Hmm, how do we extend this so it works if there is a current in the water?" And it turned out to be both possible to solve analytically, and that nobody had done it before. Nobody had thought it was possible to do. It's an example of "low-hanging fruit" that had been sitting since 1887.
I think the reason why there's "lots" of LHF is that science has become so incremental, iterating on recent results. If you go back 20-50-100 years and look at the road less traveled, there's plenty of LHF, but going down that path and shaking the trees requires more effort per initial publication than most academics can afford under the current system. But if you can afford (or get lucky enough) to find that first LHF, it usually gives you enough material to work on for quite some time such that it pays off over time.
I think the reason why there's "lots" of LHF is that science has become so incremental, iterating on recent results. If you go back 20-50-100 years and look at the road less traveled, there's plenty of LHF, but going down that path and shaking the trees requires more effort per initial publication than most academics can afford under the current system. But if you can afford (or get lucky enough) to find that first LHF, it usually gives you enough material to work on for quite some time such that it pays off over time.
[0] https://arxiv.org/pdf/1401.1682v1