Goodbye, as is somewhat commonly known, is a contraction of "God be with ye." Cite
Through the middle part of the 20th century, one encounters "good-by" as an accepted alternate spelling. See, e.g.
, Robert Frost, Good-by and Keep Cold
(1920), John D. MacDonald, The Deep Blue Good-by
(1964), but also earlier Emerson, Good-by
(1899) and plenty other examples
on Google Books.
I don't have OED access, sadly--Fowler's suggests that "good-by" is an Americanism, but doesn't add further commentary. This is consistent with the examples above.
When did this spelling fall out of general acceptance? People may still be using it, but MS Word, for instance, thinks it's a misspelling. Was there a style/usage guide or dictionary published in the 1960s/70s that was a fulcrum for change in orthography, or is this just the product of gradual development languages are always undergoing?