If we are talking about XHTML as XML, then I would argue that accessibility is already severely compromised as the medium is inappropriate for the message - ultra-strict X(HT)ML is too limiting in terms of error-handling. If we are talking about XHTML syntax served as HTML (the overwhelming majority of XHTML content) then the presence of absence of trailing slashes has no influence whatsoever on accessibility. It does however affect validity.
Same goes for other "errors" such as uppercase tags in a declared XHTML document, pages with missing or ill-formed doctypes (to the extent that the absence of a doctype does not affect page rendering), the occasional missing end tag such as
</p>. However, unless you are certain, why take the risk? Markup validated to a recognized, recent standard such as HTML 4.01 or XHTML 1.0 has the best chances of lacking parsing errors which might degrade the accessibility of the document.