Whomever does the compliant would either need a ton of liability insurance, or would need contract language that actually gets them out of responsibility (similar to the language in many travel insurance and point-of-dale service plans). The former would be expensive, so you’d probably end up with the latter.
Web accessibility compliance does require a human to look at (and interact with) a site, and it requires people who either have training, or experience. That costs more. I really hope we don’t end up with “Uber for accessibility compliance”, with testers who aren’t paid enough, and who don’t get enough time to test a site.