It was moved to the POST request that SharePoint creates while redirecting via appredirect.aspx. So, the only one way to get Site Logo Url is to handle appredirect POST request.
To initiate redirect you should use this code snippet:
Response.Redirect(TokenHelper.GetAppContextTokenRequestUrl(sharePointHostUrl, Server.UrlEncode(targetUrl)));
ContextToken, SiteLogo, Url, Title and so on can be found in the POST FormData.