The term has precedents since at least the 1950s,including the concept of the military–industrial complex, which posits a cabal of generals and defense contractors who enrich themselves through pushing the country into endless wars.
Opinion polling done in 2017 and 2018 suggests that approximately half of all Americans believe in the existence of a deep state in the United States.
It's not a myth - just like Antifa is a real group.
Tufts University professor Michael J. Glennon stated that President Barack Obama did not succeed in resisting or changing what he calls the "double government" and points to Obama's failure to close Guantanamo Bay detention camp, a major campaign promise, as evidence of the existence of a deep state.
In a 2017 interview several weeks before Trump was inaugurated, Senate Democratic Leader Chuck Schumer called Trump "really dumb" for having repeatedly criticized the CIA, saying, "Let me tell you, you take on the intelligence community, they have six ways from Sunday at getting back at you." Various commentators, as well as the ACLU, have pointed to this statement as evidence for the existence of a deep state.
Rebecca Gordon, a teacher and author at the University of San Francisco, wrote in a 2020 piece for Business Insider that Trump has used the term "deep state" to refer to the U.S. government, in particular government Institutions that "frustrate" him, as well as block or fail to implement his government policy such as courts, the Justice Department, and the news media.
Knowledge is power!