The 'four' condemned forms of marriage in the Dharmasutras and Dharmashastras were most likely practised by whom?

Those who did not accept Brahmanical norms.

Despite the claims of universal validity made by the Brahmana authors of these texts, real social relations were likely much more intricate. The vast regional diversity across the Indian subcontinent and the challenges of communication meant that the influence of the Brahmanas was not all-encompassing.

Interestingly, the Dharmasutras and Dharmashastras recognized as many as eight forms of marriage. While the first four were regarded as "good," the remaining four were condemned. It is plausible that these other forms of marriage were practiced by those who did not adhere to Brahmanical norms.