It is only described as an adjective when used before the noun, like most adjectives.1849 Macaulay Hist. Eng. I. 246 He had passed several years in diplomatic posts abroad.
2007 Vanity Fair (N.Y.) Apr. 185/2 English comestibles, the sort of thing that Englishmen abroad are supposed to yearn for.
I leave it to a grammarian to explain that.2007 South Bend (Indiana) Tribune (Nexis) 25 Apr. b3 While dean, he inaugurated a popular abroad program in which second-year law students studied in London for a year.
My explanation for this oddity is that somebody on the South Bend (Indiana) Tribune doesn't write very well. It may well be the name of the programme but that should be indicated - '... the popular "Abroad Program ...".popular abroad program
Not an oddity. That was but the most recent citation. This usage is recorded by the OED for a few hundred years. I hadn't seen it before and I was curious that the OED does not record "abroad" as a post-posited adjective.My explanation for this oddity
Merriam-Webster classes "abroad" as "adverb or adjective", as does Collins: