Rusev has removed all references to the WWE from his social media accounts and has included information to reach him for booking.
As noted, it was announced that Rusev would face The Undertaker in a casket match at the Greatest Royal Rumble, but he was replaced by Chris Jericho.
Jimmy Traina of Sports Illustrated reached out to WWE on the matter. The sports entertainment company stated that Rusev has not left the company and will be competing in the Royal Rumble match in Saudi Arabia instead:

 

Wrestling fans, I asked the WWE if Rusev has left the company. WWE said those rumors are not true, he is still with the company and he will be in the Royal Rumble match at Greatest Royal Rumble in Saudi Arabia. #RusevDay
— Jimmy Traina (@JimmyTraina) April 13, 2018