Is the platform Accessibility Compliant? To what level of standards? How do you ensure that sites stay Acc Compliant?

Yes, WCAG 2.1. The platform is Accessibility Compliant, however the content that a site-owner puts on his or her OpenScholar website, we cannot vouch for its compliance (for example, no document in a PDF format will be Accessibility Compliant). We can, however, encourage site owners to use certain AC tools like Site Improve that flags elements within the site that are not in compliance.
See also: Accessibility