I first encountered it in "Be mine, Hero!" "I refuse!" / Archenemy and Hero. It wasn't an isekai story, and the Demon Lord(ess) introduced four crop rotation as part of a larger plan to revitalize the agricultural sector. After that however, in other generic isekai trash that my brain refuses to remember, I've seen heroes introducing four crop rotation, winning the acclaim of the local village, and... then, nothing. At least in the former, you see the long-term effects through increased prosperity, reduced starvation and famines, etc.

I think seeing a clash of philosophies, ideas and worldviews is far more interesting in an isekai setting than simply having the MC introducing modern tech and watching the consequences unfold. If the world isn't so radically different from ours that the hero doesn't have to adapt and has no need to face the consequences of living as a modern human would, there isn't a need for an isekai premise. Case in point, Knights & Magic, where the MC is a reincarnated isekai'd modern Japanese salaryman, but his past life barely comes up at all.