Suggested testing notes.

"New AppOffline process to show users that upgrades are in process using the upgrade experience. Ideally testing for any issues across different hosting environments; local, Microsoft Azure, Amazon Cloud, etc. Concern areas include unable to complete installations or hung-sites resulting in a manual removal of appoffline.htm"

IMO it would be good if we had a standard filename like AppOffline.htm.bak or _AppOffline.htm and the system would rename that file instead of place it's own.
We have styling in AppOffline.htm(.bak) for some of our clients for upgrades.

@donker can you confirm these two path expectaions for finding/copying the file?

var appOfflineFile = Path.Combine(Globals.HostMapPath, "AppOffline", "App_Offline.htm");
        if (!File.Exists(appOfflineFile))
        {
            appOfflineFile = Path.Combine(this.appStatus.ApplicationMapPath, "Resources", "AppOffline", "App_Offline.htm");
        }