There are several passages from the Dune series that hint that either Earth was forgotten and likely destroyed.
The God-Emperor of Dune, Leto II, uses his genetic memory to recall ancient knowledge of Earth on two occasions.
This passage from Children of Dune could be interpreted to imply either it was no longer in human memory, (i.e. - forgotten) or that Earth ceased to exist.
It gave him a sense of timeless buoyancy to know that few in his universe would recall Chaucer or know any London except the village on Gansireed. St. Thomas was preserved in the Orange Catholic Bible and Azhar Book, but Canterbury was gone from the memories of men, as was the planet which had known it.
In this passage from God-Emperor of Dune, Leto II, remembers back to the time of distant human ancestor long ago, presumably on Earth.
This morning I was born in a yurt on the edge of horse-plain in a land of a planet which no longer exists.
And in Chapterhouse Dune, a Bene Gesserit reverend mother Odrade refers to an ancestral planet that no longer exists. She says this shortly after speaking of Old Earth and mentions that multiple planets no longer exist.
"Each planet has its own character where we draw patterns of Old Earth. ... Mark this place well, Miles. Our ancestral worlds are gone, but here we have recaptured part of our origins."
So what happened to Old Earth?
Side note: This makes me wonder if there are any world-busting weapons in the Dune series. I don't remember any such weapons.
Please answer with quotes from canon sources or extraneous writings by the authors of the Dune series. I prefer answers from Frank Herbert (true Dune canon) over all other writers (lesser canon).