Looks like they haven’t changed the favicon yet… https://abs.twimg.com/favicons/twitter.2.ico
EDIT: And in fact, it looks like their 404 page has the SVG: e.g. https://abs.twimg.com/favicons/twitter.3.ico
EDIT: And in fact, it looks like their 404 page has the SVG: e.g. https://abs.twimg.com/favicons/twitter.3.ico