The "abridged" Princess Bride isn't an actual thing, the "original novel" and "original author" are a fiction. Goldman used the fictitious author and his works a number of times in his novels.
That was part of his satires on the genre.

What!

I’m dying. I read this in middle school and the class literally thought there was an actual original Princess Bride. Lmao
Everything he says at the start of the book and mostly throughout in his commentaries is actually a framing device with fictional details:
The Princess Bride is presented as Goldman's abridgment of an older version by "S. Morgenstern", which was originally a satire of the excesses of European royalty. The book, in fact, is entirely Goldman's work. Morgenstern and the "original version" are fictitious and used as a literary device.
Goldman carried the joke further by publishing another book called The Silent Gondoliers (explaining why the gondoliers of Venice no longer sing to their passengers) under S. Morgenstern's name.
Goldman's personal life, as described in the introduction and commentary in the novel, is also fictional. In The Princess Bride, Goldman claims to have one son with his wife, a psychiatrist. In reality, Goldman has two daughters, and his wife is not a psychiatrist.
There's a number of novels that have done this. Can't remember the name but there was a, I think, sci-fi one where the author claimed to have found lost manuscripts of a once super popular but now unknown author's grand masterpiece, but not only was the author slowly degrading with dementia, but his drafts are all written in a personal shorthand. None of this is true, and the main point of the satire was to poke at authors who write these grand universes and lore to the detriment of actually ever writing a compelling story to take place within them.