It has been moved to a POST request that SharePoint creates when redirecting through appredirect.aspx. Thus, the only way to get the website logo URL is to process the appredirect email request.
To initiate a redirect, you must use this piece of code:
Response.Redirect(TokenHelper.GetAppContextTokenRequestUrl(sharePointHostUrl, Server.UrlEncode(targetUrl)));
ContextToken, SiteLogo, Url, Title, etc. can be found in the form POST FormData.
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