In case others are looking for information on how to achieve this. Works on ADFS 4.0 - Windows 2016 Server. Add to the onboard.js
file.
// Code change the Sign out page
var signoutArea = document.getElementById('signoutArea');
if (signoutArea) {
var logoutHeader = signoutArea.querySelector('#openingMessage');
var logoutWarning = signoutArea.querySelector('#instruction');
if (logoutHeader) {
logoutHeader.innerHTML = '<h2 id="logout-header" style="font-size:24px;font-weight:bold;color:#000;">You have been logged out.</h2>';
}
if (logoutWarning) {
logoutWarning.innerHTML = '<p id="logout-warning" style="margin-top:20px;font-size:16px;color:#000;">Please close your browser window.</p>';
}
document.title = "Logged out";
}