This detectBrowser.js is the same file since version 5.1.

*"# cat /var/opt/silvertail/srv/nginx/html/js/detectBrowser.js *
function oldIE() { 
var ua, ieVersion = -1;

// Do not use jQuery's $.browser because this property is deprecated in jQuery 1.3, 
// and its functionalities may be moved to a team-supported plugin in a future release of jQuery. 
// In addition, since Chrome (or/and other browsers) is able to "fake" IE, if UX team likes to over kill it, 
// check ($.support.leadingWhitespace === false) to validate this property is not available to IE 6-8 but available to other browsers. 
if (navigator.appName === 'Microsoft Internet Explorer') { 
ua = navigator.userAgent;

// If Internet Explorer, check version number 
ieVersion = /MSIE ([0-9]

{1,}
[\.0-9]

{0,}
)/i; 
if (ieVersion.exec(ua) !== null && ieVersion.exec(ua) !== undefined) { 
ieVersion = parseFloat(RegExp.$1);

if (typeof ieVersion === 'number' && !isNaN(ieVersion) && ieVersion > -1 && ieVersion < 10)

{ // all versions of IE before 10 are unacceptable. return true; }
} 
}

return false; 
}

if (oldIE())

{ window.location = "unsupported_browser.html"; }
"

Because the JavaScript is searching for the older IE 11 user agents, it redirects the session to the unsupported_browser.html, even if the browser is IE 11.