Well, as you probably guessed, this was nothing more than a tease. Dave Meltzer of Wrestling Observer Radio (h/t Ringside News), explained that this was just "website stuff" done for "fun."
Meltzer also added that the match won't happen because "Sting cannot get cleared nor is he going to attempt to get cleared nor do they want him to get cleared." So there you have it. The Sting vs. Undertaker match only remains a possibility in our imaginations.
It should be noted that the 60-year-old Sting has refused to fully close the door on a potential return to the ring. At the Starrcast event, The Vigilante said he would come back for" the right price."
And in a recent interview with Wrestling Travel, Sting admitted that Undertaker is the only wrestler he would "come out of retirement for now." The WWE Hall of Famer himself refuses to let the dream match vs. Undertaker die, but if he can't get cleared, then there's just no chance of it happening.
It should be noted that Sting and Undertaker did actually wrestle during a WCW event back in 1990 - before the latter ended up in WWE. Undertaker was performing under the ring name "Mean" Mark Callous," but this was obviously before The Phenom emerged as a WWE legend. Thus, many fans and pundits refuse to acknowledge that we've had a legitimate "Undertaker vs." Sting match, and it'll stay that way until/unless they ever go toe-to-toe in the WWE ring.
What This Means
Fans will continue to hold out hope that Sting vs. Undertaker may one day happen, but the time to do this match may have passed years ago. Sting is 60 and had to retire in 2016 after suffering a neck injury. But again, he refuses to to shut down the possibility of going up against Undertaker, so fans can have a glimmer of hope to hold on to.