That's not new, that's how domestic rabbits came to be. The veggie and pellets diets are the new stuff (with great marketing), not a grass and hay diet. I mean, just one or two generation ago here rabbits were a cheap and easy to raise common staple food.
But one thing is clear: Not every pet rabbit owner has the opportunity to feed grass and forage, you need quite a lot of it. I forage wherever I can, the 1500m² I have are not enough to feed all my rabbits, not if i make hay for the winter. (Well, at least when things go wrong like last year and way too many mouths to feed)
Hay is the next best thing to fresh grass and weeds, since some stuff in it may detoriate when being dried veggies and pellets are an addition to round the diet off, but they don't really need it, not much of it anyway. Many pet rabbits are fed a pretty rich diet.
Those industrial farmed, single species hays like Timothy and so are pretty much unknown here, we just have whatever grows on the meadows.
The risk of bringing in desease was already mentioned. Those deseases are new, they were not present some decades ago, so "back to the roots"" is not the same thing as it was back then.