Rey Mysterio is reportedly not under contract to WWE right now. The following was noted by Dave Meltzer:

"Dave Meltzer has confirmed that Mysterio's WWE deal, which he signed in September 2018, has expired but is still working for WWE. Mysterio and his son Dominik have been heavily featured on Raw in a key storyline with Seth Rollins."

Mysterio was scheduled for last week's taped episode of Raw but WWE decided to have him appear from their home with his son Dominik.