Please make plans. application/xhtml+xml has been around for at least 15 years and is a very secure way to serve web content because it causes browsers to require well-formed content.
Many if not most XSS attacks rely upon mal-formed html that slips past server-side filters that the browser compensates for. Sending content with this mime type takes away that attack vector.
W3C validates pages sent with this mime type. Twitter really should consider fixing its crawler, because quite simply, it is broken.