The technique of cramming two words into one hyphenated word like "couple-colour" and "chestnut-falls" is not a recent development. Shakespeare used such words frequently, and so did later Romantics like John Keats. But this technique came to be associated with the innovative sonic experimentation of 20th-century modernism, as expressed in the work of authors like James Joyce, William Faulkner, and others.
Because of his innovative technique and use of language, Hopkins is sometimes considered a "modernist" or "proto-modernist" poet. But it doesn't really make sense to call Hopkins a modernist because he wrote his poems well before the turn of the century. He belonged to no established school, and hyphenated words were just one of the tools of his own brand of "sprung rhythm."