Sting hasn't wrestled since 2015 when he was injured during a match with Seth Rollins and neck issues are the reason why he hasn't been cleared but he has told people that he feels fine and he would like to wrestle The Undertaker for his retirement match.
At 61 years old, it seems very unlikely that WWE would allow him to step in the ring with his neck issues but the doors are open now that WWE is doing cinematic matches that are less grueling on the body. Also, Daniel Bryan and Edge have been able to get cleared after they thought their careers were over. In Edge's case, he had to retire due to severe neck issues and Bryan's issues were concussion-related.
If the match with Taker is going to happen then it means that Taker would be stepping out of retirement after WWE confirmed last week on SmackDown that he was done as an active wrestler.
During chapter 5 of "Undertaker: Last Ride," the "never say never" line was used by Taker and he said that he would return if Vince McMahon called him.
Many fans are taking the tweet below to be a tease from Sting on an in-ring return. For what it's worth, there have been no rumors coming out from WWE's side on interest in getting Sting to wrestle again but plans can always change, especially if Vince McMahon feels a big hook is needed for a future pay-per-view.