Rey Mysterio popped into the Royal Rumble match without any kind of deal with WWE in place. But after he broke the internet by being at the Rumble there was immediate interest in bringing him back to WWE.
Mysterio has been talking to Triple H about a contract but at this point, Rey is telling people that he won’t be wrestling at Mania. That is very specific wording to say “wrestle” but the Wrestling Observer Newsletter also noted that in the cases of those surprise entries during the Women’s Royal Rumble who said that they hadn’t been contacted by WWE, “things can change on a dime”
It is also noted that Mysterio is still set to wrestle for New Japan going forth for a few matches and he’s also got some indie dates scheduled during the Summer. He will likely also be working the Cow Palace show for NJPW in July.
But never say never in WWE because Mysterio’s made surprise appearances before with WWE and both sides were able to keep a secret pretty well.