Meta tags in the .opf file can be used by online retailers to identify the accessibility features of the product. That is precisely why meta tags in the OPF were added. See
https://www.w3.org/Submission/epub-a11y/
Also note that WCAG 2.0 is not just a W3C recommendation, it is an ISO standard and has been since 2012. See
https://www.iso.org/standard/58625.html