In the past during scheduled Phabricator downtime, all requests got redirected to a wiki page. This meant that when I was going through some mail and clicking on various links, after switching to my browser I was presented with dozens of tabs with the same wiki page and, if I could remember what links I clicked, I had to go through the same process again after the downtime ended.
Instead, during scheduled Phabricator downtime, for example clicking on T2001 should bring up a page with the URL unaltered that says: "Phabricator is down for maintenance; see [[wiki page]] for details." so that only active user input fiddles with the URL. Optionally, the information could include the estimated end of the maintenance window.