It is a belief among some modern Hindus that the Upanishads in general are not part of the Vedas, nor are they Shruti (apaurusheya), and that they were authored after the Samhitas, which are the truly revealed Veda. Moreover, some even say that only the Rig Veda Samhita is the true Veda.
The first person I know of to promulgate this idea was Hindu-reformist Dayananda Saraswati.
Was there any scholar before him to promulgate the same idea? As far as I'm aware, all orthodox Vedic scholars considered the entire Veda shruti.