Origins
Castaneda's largely fabricated Don Juan books (from 1968), Michael Harner's 'core shamanism' (1980) abstracting a universal method from cross-cultural material, and the wider New Age publishing market built the movement.
The core shamanism claim
Harner argued a universal method could be extracted and taught in workshops, stripped of culture. Critics argue the culture was doing the work, and that the abstraction removed accountability, community and consequence.
Taking it seriously as its own thing
Millions of people practise some form of this. Studied as a contemporary Western religious movement it is genuinely interesting. Presented as Indigenous tradition it is a misrepresentation.