To get this feature of sign in as different user back, we can follow either permanent or temporary solution:
Permanent Solution for all web applications
1. Locate the following file and open it in a text editor: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx
2. Add the following element before the existing "ID_RequestAccess" element:
<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser" Text="<%$Resources:wss,personalactions_loginasdifferentuser%>" Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" MenuGroupId="100" Sequence="100" UseShortId="true" />
3. Save the file.
One time Temporary Solution
Append /_layouts/closeconnection.aspx?loginasanotheruser=true after site url in browser's address bar.
Example: http://sp-dev-srv:7000/_layouts/closeconnection.aspx?loginasanotheruser=true
Permanent Solution for all web applications
1. Locate the following file and open it in a text editor: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx
2. Add the following element before the existing "ID_RequestAccess" element:
<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser" Text="<%$Resources:wss,personalactions_loginasdifferentuser%>" Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" MenuGroupId="100" Sequence="100" UseShortId="true" />
3. Save the file.
One time Temporary Solution
Append /_layouts/closeconnection.aspx?loginasanotheruser=true after site url in browser's address bar.
Example: http://sp-dev-srv:7000/_layouts/closeconnection.aspx?loginasanotheruser=true
