The Oscar-winning writer and director took to X Monday ... saying the hit film based on the Broadway musical is one of the most radical big budget Hollywood films he's ever seen -- claiming it's "about radicalization in the face of careerism, fascism, propaganda."

He likens the film to other progressive movies -- like "The Bridge on the River Kwai," "The Sound of Music," "The Searchers" and "Citizen Kane."

Then, McKay makes a bold claim ... saying the way the U.S. is trending politically, he wouldn't be surprised if the movie is banned in three to five years -- given the way American politics is going.

Of course, many online were incredulous ... saying there's no chance any film -- especially a musical like "Wicked" -- would ever be banned.

But, McKay stayed persistent ... saying people who think there's no chance certain films or books could be banned aren't seeing the full picture.

"Wicked" has resonated with audiences since coming out in later November ... grossing nearly $600 million internationally -- and, part 2 of the story is set to come out late next year.

Just like the musical, the film deals with themes of racism and discrimination ... with characters prejudiced against Elphaba -- the Wicked Witch of the West -- because of her green skin.

Conservatives online blasted a British film board for the "woke" disclaimer about the movie last month ... so there's already back lash among some.