UPDATE (via F4WOnline): Dave Meltzer posted the following on his message board when this news was brought up to him.
“Anyone reporting on a Mania card now is guessing. They had a complete top six matches and everything fell apart. Everything changed again on Sunday.”
So, in a nutshell, any reports on potential matches are just guesses at this point.
There has also been no talk of Sting vs. The Undertaker happening at the event. As noted, WWE plans to have John Cena wrestle The Undertaker at the event.
Sting was last seen on WWE television in a match with Seth Rollins at the Night of Champions PPV, which saw him injure his neck.