Sting does indeed have an agreement with WWE to work and is very much in the conversation for Wrestlemania 31.
The reason we aren’t “confirming Sting for Wrestlemania” is because we haven’t heard the latest plans for the show next year. However, based on the booking at Survivor Series, the company is clearly testing the waters for a potential singles match against Triple H. Sting has made it quite clear he wants one more match, a match against The Undertaker. Such a bout hinges in large part on Undertaker’s desire. The fact “the streak” is no more will not factor into the decision for such a match.
Brock Lesnar will be working Wrestlemania next year, who unlike Sting, we can confirm for the show next March barring injury. The point of note with Lesnar is that it could be his final match with WWE if he does not agree to a contract extension. His WWE deal is up after Wrestlemania 31 and has not yet began extension talks.
As for The Rock, he has several projects lined up for next year and is a more complicated situation. Movie studios do not like Rock working for WWE because of the risk of injury delaying or derailing filming plans. However, another match is something he is interested in, it’s just unknown at this point if it will happen next year. With Lesnar and Sting already on the card, he may not choose to come back next year.